The launcher starts correctly in my language. Unfortunately, the game launches in English. What can I do to change the language?

I had the same problems. You just have to set the settings from English to Polish (language="l_polish") in the folder Users/Documents/Paradox Interactive/Stellaris (in my case in the Microsoft Cloud) and in the Settings file. Then you should be able to play the game in Polish as usual.

This means that a solution to the problem exists (unfortunately, GOG support hasn’t responded to the ticket). Which specific file did you edit? I’m not using Windows; the game is installed on a Linux system, so the file path is completely different. I assume it would be /home/marder/GOG Games/Stellaris/game/, but I don’t see a file named "settings" there.

Yes! You've hit the mark exactly! Everything is now working as it should. Thank you both very much.