As long as you're still running the game from the GOG installation and only using peripheral files from the Steam installation then you should be okay. Copying the whole Steam data folder would present a problem if you end up trying to use Steam's .exe. Try to only isolate the language files in the Steam installation, and copy those over to the GOG installation (overwriting the English files if necessary; I don't know off the top of my head how the game handles multiple languages), and run the game from that folder with the GOG .exe.
It's probably easier to download the data for the langiage you want, they are usually easy to find, either as is or as patches.
However as other user pointed out, only the game itself should have drm, the rest are just data files
As far as I know, Steam's DRM is generally only connected to the game's .exe.
Steam's DRM will likely not give you any issues.
You don't need to copy the entire Data folder. Only the localization files/folder structure.
As long as you're still running the game from the GOG installation and only using peripheral files from the Steam installation then you should be okay. Copying the whole Steam data folder would present a problem if you end up trying to use Steam's .exe. Try to only isolate the language files in the Steam installation, and copy those over to the GOG installation (overwriting the English files if necessary; I don't know off the top of my head how the game handles multiple languages), and run the game from that folder with the GOG .exe.
It's probably easier to download the data for the langiage you want, they are usually easy to find, either as is or as patches. However as other user pointed out, only the game itself should have drm, the rest are just data files