A language is a dialect with an army and navy

While we never learn who really said this but this quote is connected to Yiddish scholar Max Weinreich.

Look at the Germans dialects. People don’t understand each other, yet there is one official variety the pupils and students are taught at the school.

Now, look at the Czech and Slovakian cases. These languages are mutually intelligible to a very high degree, but the Czechs and Slovaks have different official languages and their countries are separated.

In the 1800s, there was no official Slovak language because nowadays Slovakia was under Hungarian rule and the Hungarians had their own culture.

While national revival emerges (or the existence of very nationalism) consideration for the united language of Czechs, Moravians and Slovaks took place.

However, the development in Slovakia was different and they codified their own language. It was a matter of politics. So the dialect had created his own army and, were there a sea, an own navy.

There is no border between the Czech and Slovak languages since there is a dialectic continuum.

Or look at the US dialects. They are different from each other more than General American is different from Canadian English. But we are talking about American English and Canadian English.

If British English and American English were written according to the pronunciation, there would be two languages with differences bigger than the Czech-Slovak split.

In the 19^th century, France had many languages but they enforced the official Parisian variety while pupils were physically punished for using dialects.