In the GOG-description it says "Languages: Audio and text: Deutsch (DOS version only), English" but also "Works on: Windows (XP, Vista, 7, 8, 10) and Mac OS X (10.7)". So what does DOS-version mean in this context?

Both games were published for DOS, AFAIK also in German, so both GOG-versions have German Text?

Yes, both are available in German. Only the early windows remake called Wizardry 7 Gold (which I consider inferior to the original version) isn't.

The DOS installer is listed under Goodies/Extras in your shelf.