This achievement is not unlocked from within the game.
1. Exit the game.
2. On the steam client, Right Click on the game ->
3. Open "Properties" ->
4. Go to "Language" Menu ->
5. Select "Czech" as the language from the drop-down menu ->
6. Close the panel and Update the game ->
7. Launch the Game and there you have it. You've switched to the game's other language.