This issue might be linked with outdated/corrupted saves for the game.
You will be able to change the language of the game after removing them (you should be able to put them back in after).
- If the game was installed via GALAXY, please select the game in GALAXY, click on the icon on the right side of the PLAY button -> Manage Installation -> Show Folder
- If the game was installed via offline installer, then you will have to manually navigate to the game installation folder, by default it's: C:\GOG Games\game-title\
(Cloud saves could be a factor here as well, you might have to disable them temporarily during the whole process.)
To do this, go to the Game in Galaxy >Press on the button next to the PLAY button (on the right) >Manage installation>Configure and here please uncheck "Sync local saves with GOG GALAXY cloud".
>>>The drop-down list might appear empty but the language options are still there. Simply select any "invisible" language and click on apply. This should refresh the language list and make them visible again.