For PHP libraries such DataModel, all the overhead of the translation setup is not worthwhile just to have the thing on Special:Version be translated. Indeed, a good portion of these libraries don't even add anything to Special:Version to begin with.
Components with messages should of course have translatewiki support. As you mention, this can be done by syncing the GitHub repo with the WMF one, as done with for instance Maps. I suggest filing issues for those where this is not done yet. Of course if we want to avoid new repos being created and them not being set up in this way because it's such a hassle, then the obvious thing to do is to make translatewiki not bound to the WMF git repos.