So i am well aware of the "Audio and text: English. Text only: French, Italian, German, Spanish." notice on the GOG page and know this game well. My retail version has complete german voices for the entire game. Now the GOG version however doesnt offer any language settings and strangely gives me german subtitles to english voices in realtime sequences, but playes videos with german voices.
How could that happen?

I had the same problem. The low resolution videos play with German audio, but the native resolution sequences play with English audio and German subtitles, and there are no menu options to change the language settings.

If you still have your retail version of Sudeki, do what I did:

- Go to your Sudeki game folder
- Rename the Sound folder to something different (like "SoundEnglish")
- Copy the Sound folder from your German retail version into the game folder
- Play

This worked for me and now I have German audio with German texts.

GOG devs Team is aware of the issue, i sent them a ticket , but no ETA for a fix or whatever.

well there's a problem anyway : since there's only voice-overs in english for the game itselt (GOG version)

actucally the best solution would be to strip the non-english cutscenes (short term solution), OR getting the assets for others languages (in game) like the solution offered above me.

Nevertheless , if the game is installed on a computer where the language is other than english and is part of the languages (subtitles) supported by this release, => the result is a mix-up between cutscenes & the game itself in term of voice-overs : because voice-overs will be always in english.

I went for the replacement of the english voice files with german ones from my retail disc. Which wasn't as easy to do as someone might think. Because on the disc there are no folders for different languages, all the files of all the languages are mixed together in 2 big .cab files. So it took me a while to locate the right ones by simply testing them.

What impressed me after all is how terrible the english dubbing is compared to the german one. The english voices are so far off lips as the moon from earth, while the german voices simply sound more appropriate and are well synched.

Also all the videos do have multiple audio channels for different laguages. Why that remained while all the other language files got removed remains a mystery.

The french voice-acting made me cringe.
Until I watched the videos in english on YouTube.
And that was even worse!
Now that I saw how awful the english voice-acting is, I appreciate more the not-so-bad-after-all french voice-acting!