Those games were distributed in Poland with Polish subtitles. When will this be available here (the same as with Syberia games, which is available in full polish localization on GOG)?

Excatly, where is polish subtitles?

They could just add a simple tool for modding in language packs. Then if you can locate the discs you can share them around without GOG needing to track down the licence. And for unsupported languages we could record or TTS new files and inject them with the tool. But the tool needs to come from GOG or the publisher.