African_wildlife: I'd estimate that over 90% of GOG users understand English "reasonably" well.
That is absolutely beside the point. These foreign language versions exist, they can be easily acquired on other platforms and I see no reason for them not to be included on gog. The main reason I include these on the list is to give publishers a quick hint: Hey, just mail that Wetransfer-file to gog and you're good to go!
There currently is a bit of a controversy underway on the German subforum re the missing German versions of Daedalic games. Daedalic flat out REFUSES to put their German versions on gog, first citing legal issues with the German retail distributor (who, as it turned out, has nothing whatsoever to do with digital distribution) and then claiming that they can't provide gog with these builds due to "lack of manpower" (whatever the hell they mean by that).
Seriously, how can anyone not think that this is treating gog customers as second class citizens?
African_wildlife: Regarding leaderboards, implementing support for the GOG version could necessitate building their own service from scratch (when the developer has used Steamworks features) and the system may not be able to include Steam users due to legal reasons.
No matter the reason, gog users are missing out. That's why I'm inclined to include them.