I think you mean "Slavic languages". Slovakian is a Slavic language. I'm sorry I have no answer to your question tho.

There aren't many reliable sources on the subject, but looking at the various Polish texts I could find, you could maybe go to the 1700s, go earlier than that and it's guesswork territory, a Serb or a Russian would probably have an easier time talking to a Pole from the 1500s.

A Czech speaker might understand a compatriot from roughly two hundred years ago with only fairly minor difficulties, if any, and could possibly understand – albeit with some help or perhaps only when reading a transcribed text – someone from over twice that long ago, give or take a century.

Proto-Czech and Early Old Czech spanned a period from about the 10th century to the 13th century and both would be extremely difficult bordering on outright impossible for a present day speaker to understand. Some of the words are similar or familiar in the same way one might recognize a word or two in another Slavic language, however even these would require a transcription in the first place, as both the spelling and the pronunciation were vastly different.

Medieval Czech starts to become less illegible, especially with later texts, but spelling and especially pronuniciation would still pose a challenge. The ortography reform from the early 1400s attributed to Jan Hus would prove helpful to a 21st century person, before the introduction of diacritics the language might remind a reader of Polish with its use of digraphs ("cz") instead of diacritics ("č"). Certain transcribed texts could be highly compatible with today's language, compare "Dřěvo sě listem odievá, slavíček v keřku spievá" and "Dřevo se listem odívá, slavíček v keříku zpívá".

The following centuries would see a present day reader gradually achieve a greater understanding of the language, although Habsburg rule and the Germanization policies did make the development of Czech more difficult. Spelling would still be very different to modern Czech, but for example the first complete local translation of the Bible, the Kralice Bible from the late 16th century, is highly comprehensible with a modern transcription.

For a perfectly understandable form of Czech without any major obstacles to someone from our time, look to the culmination of language development during the Czech National Revival in the 18th and 19th centuries. Josef Dobrovský and Josef Jungmann and their works on Czech grammar and vocabulary formed the basis of the language as we know it today, although the structure of the texts can obviously be sometimes rather archaic.

the earliest written records of czech language are dated to the 10th century AD.

although that language bears influences of the old slavic (which in turn has macedonian origin), it is, let's say, so-so understandable to modern readers.

to pull a wild example, it's as if a british bloke got to read accounts written by a latino gangsta somewhere in SC LA. the structure is whacked, some words are a total mistery, but you get the vibe and overall meaning.

so i think you could get by when travelling back in time to at least around 1100 AD. it would be quite difficult, but if you stuck to basic concepts, you should be fine.

Ancient venetians were populating different parts of europe as far back as 1500 B.C.. The examples of their language that they left behind can be in many cases understood by Russians, Czechs, Slovenians and other Slavic nations. Bear in mind that most most of the history from this uncertain era was written around the two world wars, when any Slavic significance was out of the question out of nationalistic interests. Lately we are getting to see quiet a few interesting results from many parts of Europe and different fields of investigation that when you look at old language found in any parts of Europe, there is a noticeable resemblance with Slavic languages much more then any other group.