Zainstalowałem D2 Rise of the Elves i niestety nie mogę zmienić języka na polski - w GOG Galaxy jest napisane, że mogę zmienić to tylko z poziomu gry, a z poziomu gry nie ma takiej opcji xD co teraz?

For reference
https//www.gogdb.org/products?search=disciples
From unofficial GOGDB
http://www.gogdb.org/product/1207663713/build/56038483226904991

Latest update on GOG Galaxy are broken. And worse they erase legacy builds. Erase and not just hide to public. In Galaxy API (public) WINOS builds link lead to just 1 public build and 0 private.

And that one last build although restore some missing files lost few folders (empty). Maybe lost some files too. K didnt examine it too deep.

As last one it is also missing Polish localisation. So in short: there is NO langauge available for GOG Galaxy version.

Use GOG (aka offline) version instead.
Go to game page and press MORE-DOWNLOAD BACKUP INSTALLERS.
Use Galaxy to download it. Or retular web browser.

If you want you can keep Galaxy build. Check game files in client - better twice in a row (Galaxy not report validation result). Then start it once directly from Galaxy.

Then you can go to download exe and instead of installing it you can use latest InnoExtract with --gog argument (use link manually created)(refer to InnoExtract tool documentation) to extract files without installing.
There is few files in temporar folders need to be manually edited and replaced. But yyou need just localisation files. I try to list them here...

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!!! Reminder !!!
Disciples II The Rise of the Elves is not Disciples 2.
So you need less files to translate in favor of completing base campaign stories in Disciples 2. You can replay them in D2ROTE too but it can be reduced size of localisation pack to exclude files for it. Audio files. Better exclude text but it require manually restoring DBF.

Here is general list for all 3 games: D1, D2(aka D2GR) and D2ROTE
http://steamcommunity.com/app/281450/discussions/0/3061871374466181591

You need Scens, ScenInfo, Interf, Sound(+SoundsP) folders, Music for 8 files, Videos, Campaign(*Note)(but no Export). Look to one of latest mine messages there for Rise of the Elves related.

Since ENG build broken and miss some files. First check did it have missing files and folder by fompare it to your Polish build. There is at least MusicP and SoundP folders missing. Compare manually each sub-folder or use Total Commander/Norton Commander.
After restoring missing files by copy-paste WITHOUT overwriting copy-paste listed localisation files overwrite original.

Keep else files original. In Interf you need only TApp.dbf and TApp.dlgwith mft fonts files.
You no need Export map.

*Note
UPD: Also Polish did not get The Alliance Crusade DLC localised so no point copy Campaign folder as well.