You mean on the fly? No, I don't think this is currently supported.

For the fan translated version (English), there is a way to revert back to the Polish version without reinstalling the game. But it would require editing the prince.ini file to add a section for the polish version and then creating a different shortcut to launch the Polish version of the game.

In that case you'll get differently named save game files per version, but if you rename them to match the other language, the save games should be compatible and appear on your loading screen.

Hi, I would like to change the language to Russian (something the store page has lead me to believe the game supports.) How do I do this? I can't even open the menu. Help!

Figured it out:
You have to go into Settings/Manage Installation/Configure and select a different language. This will require a download (I was downloading something else so it didn't work the first time)