Folks! Don’t make a mistake of learning the Slovak language first! Learn Czech instead, and then understand all the necessary phonematic and some (very occasional) structural differences.
Both nations understand each other without any difficulty.
Btw, the peaceful 1991 desintegration of Czechoslovakia was possible because there were no attempts from any side to “find scientific evidence” that either language is “primary”, the other being a “dialect”. This mutual equality helps even now maintain perfect relations, so that you still feel they live in a unitary state.
The situation is drastically opposite when we think about what happened in former Yugoslavia, where language questions (alongside identity) were the cornerstone problems causing disputes and wars, and having repercussions even now.
27 December 2017. – Brno (Czechia)