Czech language

Monokl91  What language options did you see when you installed Sims 4?  It's not possible to change the language after the fact, at least not officially, and even the not-officially-supported registry edit will only work if you have the ability to play in that language in the first place.  If you're not sure and don't mind reinstalling the game, the option to change the language shows up when you're about to confirm the download, on the same page as the install location and the ability to select your packs.

Saves are stored locally on your computer, not on EA servers.  If you still have access to the old computer, you can retrieve your saves and other user data, stored in Documents > Electronic Arts > The Sims 4.  You could copy over the entire folder, but it would be best to use only the files you'd like to keep.  For example, transfer only the files within the saves folder, not the folder itself.