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.
