Is there any way to switch the in-game language to English? (It's in German.)
There don't appear to be any language options in the game.

Ah yes, there's the language setup, thanks. But when I click on it in Galaxy, it doesn't do anything.
I can start the language_setup.exe in the TWAU directory (needs admin rights) but no matter what option I choose (I tried all languages), the game stays in German.
I even checked the registry entry, which changes correctly. But the game seems to just ignore it.

Sounds like you are going to need to contact support.
If you do, be sure to share their solution for anyone else who might have this problem in the future.
Best of luck!

Alright, I contacted the support team. So hopefully the language can soon be changed by using the language setup. This thread has two workarounds:

By setting the language to English for non-Unicode programs (Gebietsschema ändern) in the control panel (Systemsteuerung) - Region and Language (Zeit, Sprache, Region) - Administrative (Verwaltung), TWAU uses English texts. And that's the one I'll use for the time being.

Otherwise you could download SBAppLocale and set the locale to e.g. US (I haven't tried that myself but it seems to work).

I'll post again if there are news from the support team.

(I've marked the post from WildHobgoblin as the solution, because that seems to be the way to go right now.)

UPDATE:
The GOG-support advised me to set the language setting for non-unicode programs, as noted above.
This is the link they provided me with: eightforums

but then why did they put in game folder a "langage_setup.exe" that popup a window with radio buttons for english, german, french and dont remember the 4th ?

it didnt work for any of you ?

Ridiculous GoG release in my opinion, on Steam we just install and it's setup in the language we want, on GoG we have to change Windows Region settings.

Because of stuff like this, that GoG will always be the underdog.