I think, it is rather organizational than technical issue.
Drakensang: The River of Time and Phileasson's Secret are published on Steam by US publisher Cosmi corporation, which has acquired ValuSoft after THQ bankruptcy.
Only English versions are available. And these games are region-locked in some countries ( I'm not sure where exactly, because I checked using different circuits in Tor browser ).
The original Drakensang is published by DTP Entertainment itself. It is available in German, English and French ( same as on GOG ). And it seems to be region-free.
I assume, that DTP Entertainment does not have the rights for English localization
. And Cosmi does not want to publish these games anywhere, but on Steam.
However, it seems that French publisher Micro Application had published English localization of Drakensang: The River of Time ( DVD release ).
If it is a different localization, it could possibly be licensed by either DTP or GOG instead of Cosmi version.
As for technical problems, Radon Labs had open-sourced Nebula Device 3 engine
, which is used for all their Drakensang games: https://code.google.com/archive/p/nebuladevice/source/default/source
Therefore, it can be fixed and optimized to run all Drakensang games on modern operating systems. It will take a lot of hard work, but if enthusiastic fan community will organize, it is absolutely possible.
In this case, we will be able to play DRM-free versions of Drakensang: The River of Time and Phileasson's Secret by using open-source engine and linking resources from games purchased elsewhere.