That NWN allows you to change the language in-game would be news to me. The diamond edition didn't allow it, and I don't see any big changes in NWN:EE's option menu to suggest that you can do it now. I'm pretty confident you have to actually install the game in German.

Now, I don't use Galaxy, so I have no idea how you can tell it to switch installation language, but if you download the offline installers in your GOG library (at least in the browser), there's an option on the game tab to switch the language (see attached image), and the German installation file is named differently than the English one, which seems to confirm my theory.

It's true that neither EE nor DE lets you change the language in-game. However, what EE brings to the table is the possibility to change the language without reinstalling the game (and without installing any localization package).

In nwn.ini, under [DisplayOptions], you can select the language you want to use.

It will look something like this:

[Display Options]
Language=pl

Change "pl" to "de" and the game should launch in German.
Note that this doesn't change the language of the game's movies and voices - it's text only. If you installed the game in English, you will have English movies and voices and German text. For full localization, you need to download a localization package for your language (in the form of a free DLC on GOG):

https://www.gog.com/game/neverwinter_nights_enhanced_edition_german_extras