A more updated version of this paper with working hyperlinks can be found on Academia.edu here.
There is much nonsense said about the mutual intelligibility of the various languages in the Slavic family. It’s often said that all Slavic languages are mutually intelligible with each other. This is simply not the case.
Method: It is important to note that the percentages are in general only for oral intelligiblity and only in the case of a situation of a pure inherent intelligiblity test. An inherent pure inherent intelligibility test would involve a a speaker of Slavic lect A listening to a tape or video of a speaker of Slavic Lect A.
Written intelligibility is often very different from written intelligibility in that in a number of cases, it tends to be higher, often much higher, than oral intelligibility. Written intelligibility was only calculated for a number of language pairs. Most pairs have no figure for written intelligibility.
A number of native speakers of various Slavic lects were interviewed about mutual intelligibility, language/dialect confusion, the state of their language, its history and so on. In addition, a Net search was done of forums where speakers of Slavic languages were discussing how much of other Slavic languages they understand. These figures were tallied up for each pair of languages to be tabulated and were then all averaged together. Hence the figures are averages taken from statements by native speakers of the languages in question.
Complaints have been made that many of these percentages were simply wild guesses with no science behind them. This is not the case as all figures were derived from estimates by native speakers themselves, often a number of estimates averaged together.
True science would involve scientific intelligibility testing of Slavic language pairs. There has been no scientific intelligiblity testing of any Slavic language pairs that I am aware of. Obviously, in order to answer these questions in a scientific manner, scientific intelligibility testing needs to be done. The problem is that most linguists are not interested in scientific intelligibility testing of language pairs.
Serbo-Croatian (Shtokavian) has 55% intelligibility of Macedonian (varies from 25-90%), 27% of Slovenian, 25% of Slovak, 20% of Ukrainian, 13% of oral Bulgarian and 25% of written Bulgarian, 10% of oral Russian and 22% of written Russian, 10% of Czech, and 5% of Polish.
Chakavian has 82% intelligibility of Kaikavian.
Kaikavian has 82% intelligibility of Chakavian.
Bulgarian has 80% intelligibility of Macedonian, 41% of Russian (varies from 7-75%) and 5% of Polish and Czech.
Macedonian has 65% oral and written intelligibility of Bulgarian.
Czech has 82% intelligibility of Slovak (varies from 70-95%), 12% of Polish and 5% of Russian and Bulgarian.
Polish has over 90% intelligibility of Sorbian, 22% of Silesian, 12% of Czech, 6% of Russian and 5% of Bulgarian.
Russian has 85% intelligibility of Rusyn, 74% of oral Belorussian and 85% of written Belorussian, 60% of Balachka, 50% of oral Ukrainian and 85% of written Ukrainian, 36% of oral Bulgarian (varies from 7-70%) and 80% of written Bulgarian, 38% of Polish, 30% of Slovak and oral Montenegrin and 50% of written Montenegrin, 12% of oral Serbo-Croatian and 25% of written Serbo-Croatian and 10% of Czech.
Belorussian has 80% intelligibility of Ukrainian and 55% of Polish.
Ukrainian has 82% intelligibility of Belorussian and Rusyn and 55% of Polish.
Slovak has 82% intelligibility of Czech (varies from 70-95%).
Eastern Slovak has 82% intelligibility of Rusyn and 72% of Ukrainian.
Saris Slovak has 85% intelligibility of Polish.
Reactions: So far there have been few reactions to the paper. However, a Croatian linguist has helped me write part of the Croatian section, and he felt that at least that part of the paper was accurate. A Serbian native speaker felt that the percentages for South Slavic seemed to be accurate.
A professor of Slavic Linguistics at a university in Bulgaria reviewed the paper and felt that the percentages were accurate. He was a member of a group of linguists who met periodically to discuss the field. He printed out the paper and showed it to his colleagues at the next meeting, and they spent some time discussing it. No professional linguist has yet to discount the percentages in this paper. The paper seems to have gone over well in the scientific linguistic community.
Now onto the discussion.
There is much nonsense floating around about Serbo-Croatian or Shtokavian. The main Shtokavian dialects of Croatian, Serbian, Montenegrin and Bosnian are mutually intelligible.
However, the Croatian macrolanguage has strange lects that Standard Croatian (Štokavian) cannot understand.
For instance, Čakavian Croatian is not intelligible with Standard Croatian. It consists of at least four major dialects, Ekavian Chakavian, spoken on the Istrian Peninsula, Ikavian Chakavian, spoken in southwestern Istria, the islands of Brač, Hvar, Vis, Korčula, and Šolta, the Pelješac Peninsula, the Dalmatian coast at Zadar and the outskirts of Split, and inland at Gacka, Middle Chakavian, which is Ikavian-Ekavian transitional, and Ijekavian Chakavian, spoken at the far southern end of the Chakavian language area on Lastovo Island, Janjina on the Pelješac Peninsula, and Bigova in the far south near the border with Montenegro.
Ekavian Chakavian has two branches – Buzet and Northern Chakavian. Buzet is actually transitional between Slovenian and Kaikavian. It was formerly thought to be a Slovenian dialect, but some now think it is more properly a Kaikavian dialect. There are some dialects around Buzet that seem to be the remains of old Kaikavian-Chakavian transitional dialects (Jembrigh 2014).
Ikavian Chakavian has two branches – Southwestern Istrian and Southern Chakavian. The latter is heavily mixed with Shtokavian.
Some reports say there is difficult intelligibility between Ekavian Chakavian in the north and Ikavian Chakavian in the far south, but speakers of Labin Ekavian in the far north say they can understand the Southeastern Istrian speech of the southern islands very well (Jembrigh 2014).
Čakavian differs from the other nearby Slavic lects spoken in the country due to the presence of many Italian words.
Chakavian actually has a written heritage, but it was mostly written down long ago. Writing in Chakavian started very early in the Middle Ages and began to slow down in the 1500’s when writing in Kaikavian began to rise. However, Chakavian magazines are published even today (Jembrigh 2014).
Although Chakavian is clearly a separate language from Shtokavian Croatian, in Croatia it is said that there is only one Croatian language, and that is Shtokavian Croatian. The idea is that the Kaikavian and Chakavian languages simply do not exist, though obviously they are both separate languages. Recently a Croatian linguist forwarded a proposal to formally recognize Chakavian as a separate language, but the famous Croatian Slavicist Radoslav Katičić argued with him about this and rejected the proposal on political, not linguistic grounds. This debate occurred only in Croatian linguistic circles and the public knows nothing about it (Jembrigh 2014).
Kaikavian Croatian, spoken in northwest Croatia and similar to Slovenian, is not intelligible with Standard Croatian.
Kaikavian is fairly uniform across its speech area, whereas Chakavian is more diverse (Jembrigh 2014).
In the 1500’s, Kaikavian began to be developed in a standard literary form. From the 1500’s to 1900, a large corpus of Kaikavian literature was written. Kaikavian was removed from public use after 1900, hence writing in the standard Kaikavian literary language was curtailed. Nevertheless, writing continues in various Kaikavian dialects, which still retain some connection to the old literary language, although some lexicon and grammar are going out (Jembrigh 2014).
However, Chakavian and Kaikavian have high, but not full, mutual intelligibility. Intelligibility between the two is estimated at 82%. Most Croatian linguists recognized Kaikavian as a separate language. However, any suggestions that Kaikavian is a separate language are censored on Croatian TV (Jembrigh 2014).
Nevertheless, the ISO has recently accepted a proposal from the Kaikavian Renaissance Association to list the Kaikavian literary language written from the 1500’s-1900 as a recognized language with an ISO code of kjv. The literary language itself is no longer written, but works written in it are still used in public for instance in dramas and church masses (Jembrigh 2014). This is heartening, although honestly, Kaikavian as an existing spoken lect also needs to be recognized as a living language instead of a dialect of “Croatian,” whatever that word means.
Furthermore, there is a dialect continuum between Kaikavian and Chakavian as there is between Kaikavian and Slovenian, and lects with a dialect continuum between them are always separate languages. There is an old Kaikavian-Chakavian dialect continuum of which little remains, although some of the old Kaikavian-Chakavian transitional dialects are still spoken (Jembrigh 2014).
Kaikavian differs from the other Slavic lects spoken in Croatia in that is has many Hungarian and German loans (Jembrigh 2014). Kaikavian is probably closer to Slovenian than it is to Chakavian.
Nevertheless, although intelligibility with Slovenian is high, Kaikavian lacks full intelligibility with Slovenian. Yet there is a dialect continuum between Slovenian and Kaikavian. Kaikavian, especially the Zagorje Kaikavian dialect around Zagreb, is close to the Shtajerska dialect of Slovene. However, leaving aside Kaikavian speakers, Croatians have poor intelligibility of Slovenian.
Molise Croatian is a Croatian language spoken in a few towns in Italy, such as Acquaviva Collecroce and two other towns. A different dialect is spoken in each town. Despite a lot of commonality between the dialects, the differences between them are significant. Intelligibility issues are not known. A koine is currently under development. The Croatians left Croatia and came to Italy from 1400-1500. The base of Molise Croatian was Shtokavian with an Ikavian accent and a heavy Chakavian base similar to what is now spoken as Southern Kaikavian Ikavian on the islands of Croatia. Molise Croatian is not intelligible with Standard Croatian.
Burgenland Croatian, spoken in Austria, is intelligible to Croatian speakers in Austria, Czech Republic, Slovakia and Hungary, but it has poor intelligibility with the Croatian spoken in Croatia.
Therefore, for the moment, there are five separate Croatian languages: Shtokavian Croatian, Kaikavian Croatian, Chakavian Croatian, Molise Croatian and Burgenland Croatian.
Serbian is a macrolanguage made up to two languages: Shtokavian Serbian and Torlak or Gorlak Serbian.
Shtokavian is simply the same Serbo-Croatian language that is also spoken in Croatia, Montenegro and Bosnia. It forms a single tongue and not separate languages as many insist. The claim for separate languages is based more on politics than on linguistic science.
Torlak Serbian is spoken in the south and southwest of Serbia and is transitional to Macedonian. It is not intelligible with Shtokavian, although this is controversial.
Torlakians are often said to speak Bulgarian, but this is not exactly the case. More properly, their speech is best seen as closer to Macedonian than to Bulgarian or Serbo-Croatian. The Serbo-Croatian vocabulary in both Macedonian and Torlakian is very similar, stemming from the political changes of 1912; whereas these words have changed more in Bulgarian.
The Torlakian spoken in the southwest is different. It is not really either Bulgarian or Serbo-Croatian but instead it is best said that they are speaking a mixed Bulgarian-Serbo-Croatian language. In the towns of Pirot and Vranje, it cannot be said that they speak Serbo-Croatian; instead they speak this Bulgarian-Serbo-Croatian mixed speech.
It’s also said that Serbo-Croatian can understand Bulgarian and Macedonian, but this is not true. However, the Torlak Serbians can understand both Macedonian and Bulgarian well, as this is a Serbo-Croatian dialect transitional to both languages. Intelligibility figures for Torlakian and Macedonian/Bulgarian are not known.
Intelligibility in the Slavic languages of the Balkans is much exaggerated.
Slovenian speakers find it hard to understand most of the other Yugoslavian lects except for Kaikavian Croatian. Serbo-Croatian intelligibility of Slovenian is 25-30%.
A lect called Čičarija Slovenian is spoken on the Istrian Peninsula in Slovenia just north of Croatia. This is a Chakavian-Slovenian transitional lect that is hard to categorize, but it is usually considered to be a Slovenian dialect.
Bulgarian and Macedonian can understand each other to a great degree (65-80%), but not completely. However, the Ser-Drama-Lagadin-Nevrokop dialect in northeastern Greece and southern Bulgaria and the Maleševo-Pirin dialect in eastern Macedonia and western Bulgaria are transitional between Bulgarian and Macedonian. The Aegean Macedonian dialects mostly spoken in Greece, such as the Lerinsko-Kostursko and Solunsko-Vodensko dialects, sound more Bulgarian than Macedonian.
Russian has a decent intelligibility with Bulgarian, possibly on the order of 50% (varies from 7-75%) but Bulgarian intelligibility of Russian seems lower. Nevertheless, Bulgarian-Russian intelligibility seems much exaggerated. Some Russians and Bulgarians say they understand almost nothing of the other language. Nevertheless, most Bulgarians over the age of 30-35 understand Russian well since studying Russian was mandatory under Communism.
However, Bulgarian-Russian written intelligibility is much higher. Bulgarian and Russian are close because the Ottoman rulers of Bulgaria would not allow printing in Bulgaria. Hence, many religious books were imported from Russia, and these books influenced Bulgarian. Russian influence only ended in 1878.
Serbo-Croatian and Bulgarian have 10-15% oral intelligibility, however, there are Bulgarian dialects that are transitional with Torlak Serbian. Written intelligibility is higher at 25%. Macedonian and Bulgarian would be much closer together except that in recent years, Macedonian has been heavily influenced by Serbo-Croatian, and Bulgarian has been heavily influenced by Russian.
This difference is because Bulgarian is not spoken the same way it is written like Serbo-Croatian is. However, Bulgarians claim to be able to understand Serbo-Croatian better than the other way around. There is a group of Bulgarians living in Serbia in the areas of Basilograd and Dimotrovograd who speak a Bulgarian-Serbian transitional dialect, and Serbs are able to understand these Bulgarians well.
Serbo-Croatian has variable intelligibility of Macedonian, averaging ~55%, while Nis Serbians have ~90% intelligibility with Macedonian. Part of the problem between Serbo-Croatian and Macedonian is that so many of the basic words – be, do, this, that, where – are different, however, much of the rest of the vocabulary is the same. Serbo-Croatian speakers can often learn to understand Macedonian well after some exposure.
Most Macedonians already are able to speak Serbo-Croatian well. This gives rise to claims of Macedonians being able to understand Serbo-Croatian very well, however, much of this may be due to bilingual learning. In fact, many Macedonians are switching away from the Macedonian language towards Serbo-Croatian.
The Macedonian spoken near the Serbian border is heavily influenced by Serbo-Croatian and is quite a bit different from the Macedonian spoken towards the center of Macedonia. One way to look at Macedonian is that it is a Serbo-Croatian-Bulgarian transitional lect. The intelligibility of Serbo-Croatian and Macedonian is highly controversial, and intelligibility studies are in order.
Croats say Macedonian is a complete mystery to them.
Czech and Polish are incomprehensible to Serbo-Croatian (Czech 10%, Polish 5%), but Serbo-Croatian has some limited comprehension of Slovak, on the order of 25%.
Serbo-Croatian and Russian have 10-15% intelligibility, if that, yet written intelligibility is higher at 25%.
Serbo-Croatian has only 20% intelligibility of Ukrainian.
Slovenians have a very hard time understanding Poles and Czechs and vice versa.
It’s often said that Czechs and Poles can understand each other, but this is not so. Much of the claimed intelligibility is simply bilingual learning. Czechs claim only 10-15% intelligibility of Polish.
The intelligibility of Polish and Russian is very low, on the order of 5-10%.
Polish is not intelligible with Kashubian, a language related to Polish spoken in the north of Poland, but figures are not known. Kashubian itself is a macrolanguage made up of two different languages, South Kashubian and North Kashubian, as the two have difficult intelligibility.
Silesian or Upper Silesian is also a separate language spoken in Poland, often thought to be halfway between Polish and Czech. It may have been split from Polish for up to 800 years, where it underwent heavy German influence. Polish lacks full intelligibility of Silesian, although this is controversial (see below). Some Poles say they find Silesian harder to understand than Belorussian or Slovak, which implies intelligibility of 20-25%.
The more German the Silesian dialect is, the harder it is for Poles to understand. In recent years, many of the German words are falling out of use and being replaced by Polish words, especially by young people. Poles who know German and Old Polish can understand Silesian quite well due to the Germanisms and the presence of many older Polish words, but Poles who speak only Polish have a hard time with Silesian.
Many Poles insist that Silesian is a Polish dialect, but this is based more on politics than reality. In fact, people in the north of Poland regard Silesian as incomprehensible. 40% of Silesian vocabulary is different from Polish, mostly Germanisms. The German influence is more prominent in the west; Polish influence is greater in the east. Many Silesian speakers now speak a watered down version of Silesian which is more properly seen as a Polish dialect with some Silesian words. Pure Silesian appears to be a dying language.
Silesian itself appears to be a macrolanguage as it is more than one language since as Opole Silesian speakers cannot understand Katowice Silesian, so Opole Silesian and Katowice Silesian are two different languages.
Cieszyn Silesian or Ponaszymu is a language closely related to Silesian spoken in Czechoslovakia in the far northeast of the country near the Polish and Slovak borders. It differs from the rest of Silesian in that it has undergone heavy Czech influence. Some say it is a part of Czech, but more likely it is a part of Polish like Silesian.
People observing conversation between Cieszyn Silesian and Upper Silesian report that they have a hard time understanding each other. Cieszyn Silesian speakers strongly reject the notion that they speak the same language as Upper Silesians. Ponaszymu also has many Germanisms which have been falling out of use lately, replaced by their Czech equivalents. Ponaszymu appears to lack full intelligibility with Czech. In fact, some say the intelligibility between the two is near zero.
Lach is a Czech-Polish transitional lect with a close relationship with Cieszyn Silesian. However, it appears to be a separate language, as Lach is not even intelligible within itself. Instead Eastern Lach and Western Lach have difficult intelligiblity and are separate languages so Lach itself is a macrolanguage. Lach is not fully intelligible with Czech; indeed, the differences between Lach and Czech appear to be greater than the differences between Silesian and Polish, despite the fact that Lach has been heavily leveling into Moravian Czech for the last 100 years.
Czechs say Lach is a part of Czech, and Poles say Lach is a part of Polish. The standard view among linguists seems to be that Lach is a part of Czech. However, another view is that Lach is indeed Lechitic, albeit with strong Czech influence.
Polish has excellent intelligibility of Upper Sorbian and Lower Sorbian, possibly over 90%. Furthermore, Upper and Lower Sorbian have over 90% intelligibility of each other, so instead of being two different languages, they are dialects of a single tongue, Sorbian.
It is often said that Ukrainian and Russian are intelligible with each other or even that they are the same language (a view perpetuated by Russian nationalists). It is not true at all that Ukrainian and Russian are mutually intelligible, as Russian only has 50% intelligibility of Ukrainian. For example, all Russian shows get subtitles on Ukrainian TV. Yet some say that the subtitles are simply put on as a political move due to Ukraine’s puristic language policy. Ukrainian and Russian only have 60% lexical similarity. Polish and Ukrainian have higher lexical similarity at 72%, and Ukrainian intelligibility of Polish is ~50%+.
However, there are dialects in between Ukrainian and Russian such as the Eastern Polissian and Slobozhan dialects of Ukrainian that are intelligible with both languages. Complicating the picture is the fact that many Ukrainians are bilingual and speak Russian also. Ukrainians can understand Russian much better than the other way around. Nevertheless Ukrainian intelligibility of Russian is hard to calculate because presently there are few Ukrainians in Ukraine who do not speak Russian. Most of the Ukrainian speakers who do not speak Russian are in Canada at the moment.
In addition, the Slobozhan dialects of Ukrainian and Russian such as (Slobozhan Ukrainian and Slobozhan Russian) spoken in the Kantemirovka (Voronezhskaya Oblast, Russia), and Kuban Russian or Balachka spoken in the Kuban area right over the eastern border of Ukraine are very close to each other. Slobozhan Russian can also be called Kuban Russian or Balachka.
It is best seen as a Ukrainian dialect spoken in Russia – specifically, it is markedly similar to the Poltavian dialect of Ukrainian spoken in Poltava in Central Ukraine. Although the standard view is that Balachka is a Ukrainian dialect, some linguists say that it is actually a separate language closely related to Ukrainian. An academic paper has been published making the case for a separate Balachka language. In addition, Balachka language associations believe it is a separate language. Intelligibility between Balachka and Ukrainian is not known. Russian only has 60% intelligibility of Balachka.
However, Balachka is dying out and is now spoken only by a few old people. Most people in the region speak Russian with a few Ukrainian words.
Slobozhan Russian is very close to Ukrainian, closer to Ukrainian than it is to Russian, and Slobozhan Ukrainian is very close to Russian, closer to Russian than to Ukrainian. Slobozhan Ukrainian speakers in this region find it easier to understand their Russian neighbors than the Upper Dniestrian Ukrainian spoken in the far west in the countryside around Lvov. Upper Dniestrian is influenced by German and Polish.
The Russian language in the Ukraine has been declining recently mostly because since independence, the authorities have striven to make the new Ukrainian as far away from Russian as possible by adopting the Kharkiv Standard adopted in 1927 and jettisoning the 1932 Standard which brought Ukrainian more in line with Russian. For instance, in 1932, Ukrainian g was eliminated from the alphabet in order to make Ukrainian h correspond perfectly with Russian g. After 1991, the g returned to Ukrainian. Hence, Russians understand the colloquial Ukrainian spoken in the countryside pretty well, but they understand the modern standard heard on TV much less. This is because colloquial Ukrainian is closer to the Ukrainian spoken in the Soviet era, which had huge Russian influence.
The intelligibility of Belorussian with both Ukrainian and Russian is a source of controversy. On the one hand, Belorussian has some dialects that are intelligible with some dialects of both Russian and Ukrainian. For instance, West Palesian is a transitional Belarussian dialect to Ukrainian. Some say that West Palesian is actually a separate language, but the majority of Belarussian linguists say it is a dialect of Belarussian (Mezentseva 2014). Belarussian and Ukrainian have 85% similar vocabulary.
Nevertheless, Russian has high intelligibility of Belarussian, on the order of 75%. Belarussian is nonetheless a separate language from both Ukrainian and Russian.
From some reason, the Hutsul, Lemko, Boiko dialects of the Rusyn language are much more comprehensible to Russians than Standard Ukrainian is. Intelligibility may be 85%. Rusyn-Ukrainian intelligibility is described as similar to Czech-Slovak intelligibility – therefore, the intelligibility between Rusyn and Ukrainian is around 82%.
Rusyn-Ukrainian intelligibility is said to be the same as Ukrainian-Belorussian intelligibility, so Ukrainian and Belarussian also have ~82% intelligibility. At least the Lemko dialect of Rusyn has only marginal intelligibility with Ukrainian. Lemko is spoken heavily in Poland, and it differs from Standard Rusyn in that it has a lot of Polish vocabulary, whereas Standard Rusyn has more influences from Hungarian and Romanian.
The Rusyn language is composed of 50% Slovak roots and 50% Ukrainian roots, so some difficult intelligibility with Ukrainian might be expected. It has also been described as a transitional dialect between Polish and Slovak. Eastern Slovak has ~80% intelligibility of Rusyn.
Pannonian Rusyn is spoken by a group of Rusyns who migrated to northwestern Serbia (the Bachka region in Vojvodina province) and Eastern Croatia from eastern Slovakia and western Ukraine 250 years ago. Pannonian Rusyn is actually a part of Slovak, and Rusyn proper is really a part of Ukrainian. Pannonian Rusyn lacks full intelligibility of Rusyn proper. Not only that, but it is not even fully intelligible with the Eastern Slovak that it resembles most.
The intelligibility of Czech and Slovak is much exaggerated. It is true that Western Slovak dialects can understand Czech well, but Central Slovak, Eastern Slovak and Extraslovakian Slovak dialects cannot.
It is also said that West Slovak (Bratislava) cannot understand East Slovak, so Slovak may actually two different languages, but this is controversial. Western Slovak speakers say Eastern Slovak sounds idiotic and ridiculous, and some words are different, but other than that, they can basically understand it. Other Western Slovak speakers (Bratislava) say that Eastern Slovak (Kosice) is hard to understand. Bratislava speakers say that Kosice speech sounds 1/2 Slovak and 1/2 Ukrainian and uses many odd and unfamiliar words. Intelligibility testing between East and West Slovak would seem to be in order.
Much of the claimed intelligibility between Czech and Slovak was simply bilingual learning. Since the breakup, young Czechs and Slovaks understand each other worse since they have less contact with each other. In the former Czechoslovakia, everything was 50-50 bilingual – media, literature, etc. Since then, Slovak has been disappearing from the Czech Republic, so the younger people don’t understand Slovak so well.
Intelligibility of Czech and Slovak is 82% and varies from 70-95% depending on the dialect. Intelligibility problems are mostly on the Czech end because they don’t bother to learn Slovak while many Slovaks learn Czech. There is as much Czech literature and media as Slovak literature and media in Slovakia, and many Slovaks study at Czech universities. When there, they have to pass a language test. Czechs hardly ever study at Slovak universities.
Czechs see Slovaks as country bumpkins – backwards and folksy but optimistic, outgoing and friendly. Czechs are more urbane. The written languages differ much more than the spoken ones.
The languages really split about 1,000 years ago, but written Slovak was based on written Czech, and there was a lot of interlingual communication. A Moravian Czech speaker (Eastern Czech) and a Bratislavan Slovak (Western Slovak) speaker understand each other very well. They are essentially speaking the same language.
However, in recent years, there has also been quite a bit of bilingual learning. Young Czechs and Slovaks talk to each other a lot via the Internet. There are also some TV shows that show Czech and Slovak contestants untranslated (like in Sweden where Norwegian comics perform untranslated), and most people seem to understand these shows.
All foreign movies in both the Czech Republic and Slovakia are translated into Czech, not Slovak. Far Northeastern Slovak (Saris Slovak) near the Polish border is close to Polish and Ukrainian. Intelligibility data for Saris Slovak and Ukrainian is not known. Saris Slovak has high but not complete intelligibility of Polish, possibly 85%. Eastern Slovak may have 72% intelligibility of Ukrainian.
Southern Slovak on the Hungarian border has a harder time understanding Polish because they do not hear it much. This implies that some of the high intelligibility between Slovak and Polish may be due to bilingual learning on the part of Slovaks.
Russian has low intelligibility with Czech and Slovak, maybe 30%.
Jembrigh, Mario. Croatian linguist. December 2014. Personal communication.
Mezentseva, Inna. English professor. Vitebsk State University. Vitebsk, Belarus. December 2014. Personal communication.
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