I would like to ask how to change the game language (for example: from English to Polish)?
I'm asking because the game only offers English, and the included LangChange.exe application only has the option to change the language: English, Simplified Chinese, Traditional Chinese, and I don't know how to force the game to change to another language, and the description contains a lot of other languages that supposedly exist. in the game.

Maybe download the offline installer and change the language to polski before you download it?
Don't use galaxy so i don't know how it's there.

No, download ONLY ever English version of games as all other versions are mostly out of date.
Then after installation, start the little utility programme called Language Setup that was installed together with the game.

And I wrote about this program (Language Setup.exe), but when I connected it, it only displayed three items:
English
Simplified Chinese
Traditional Chinese
And no other languages.

Oops! Sorry, it seems I cannot read today :-(

I have the disc version of Reunion installed on this PC....so it works a little differently. But, for X3:Terran Conflict et al it works correctly and gives the whole list. Did you try removing the whole game, cleaning out the registery and redownload (it was recently updated too) and reinstall the game?

This is what it looks like when I run the language Setup.exe file (version X3: Reunion).

Now what we all want to know is whether this is a problem on OP's side or a problem with GOG's files.