You can't run Skyrim classic (or SE) and Enderal classic (or SE) in parallel. Even if they are separate games, you can only run one at a time.
So close Skyrim and start Enderal alone. There were rare problems in the past when Skyrim was not installed. That was the idea.
You can also try to start Enderal via skseloader.exe if that doesn't work.
Independently of that, we are not aware of such problems with the Polish version (you could of course test it by installing Enderal in another language like English, for example. But I don't think that's the reason).