language_setup.exe is missing in offline installer files.
So changing to e.g. german or french isn't possible.

language_setup.exe is included when installed online over galaxy.

language_setup.exe is a utility that GOG created. They recently integrated the language selection for the game into Galaxy, so maybe they forgot they should still include it in the offline installers since there you don't have a launcher for this functionality. Please contact GOG support about this.

allready done (support ticket opened)

A solution meanwhile:
go to [GOG folder]\Divinity - Original Sin 2\DefEd\Data\Localization
open language.lsx (it's text file like xml)
change
<attribute id="Value" value="English" type="20" />
into desired value (foldername of language, e.g. "German") and save.
No config tool needed.

Other research results:
it's possible to delete all folders in [GOG folder]\Divinity - Original Sin 2 except DefEd
(bin / classic / Data)
all files in root can be deleted too.
Content of DefEd (bin and Data) can be copied into root of game.
In Bin EoCApp.exe is the classical game.exe

So I have cut down filesize from 55 GB to around 21 GB (removed classic edition and launcher),
only left definitive edition.

I don't get it why I should download additional xx GB which I nearly never use (why start classic when definitive is latest).
It would be better to split downloads for both editions.