I doubt a migration to a more used platform will automatically attract new contributors.
Over the last year, we've moved m2e and Tycho to GitHub. The results are outstanding: both projects gathered some excellent contributors who've been taking care of things the former committer base wasn't capable (by lack of time or competencies) of handling; and some now people have starting doing smaller yet useful contributions.
Maybe it's a coincidence; but my impression, working on multiple projects, is that the ones that moved to GitHub seem to have benefited from a very important community boost.
FWIW, I used to have my doubts just like you and wouldn't have welcome the idea of migrating Platform a few years ago; but recent experienced really changed my mind here.
I perceive the committers community (language barrier, mindset, soft skills, empathy and available time for other people’s contribution, response time) as a bigger obstacle then the technical barrier.
That's true, but it doesn't really make moving to GitHub irrelevant.
And actually, the issues you're mentioning are one step after: it's for people who actually start contributing. GitHub wouldn't affect that step very much, it's the previous step ("how can I contribute? Oh Gerrit, that's too hard") that becomes simpler. So the expected result would be that the project receives more contributions; but it's not expected that issues like long response time would not be fixed magically by this move.
Will github solve nitpicking? I doubt it will by its own.
Clearly not; but again, it's not the goal.
Please make sure PDE’s „Import from Repository“ works with github.
Doesn't it already? If not please open a bug.
Does migration work? I already tried to clone jdt on github but it failed with various errors.
No, the current GitHub repos are mirrors (which is confusing) and aren't expected to be used for development; at least they're not supported by the project.
Does Github mean eclipse does vendor lock in to Microsoft or will there be a neutral backup / possibility to migrate back?
Source code is Git. So it's portable and IIRC the Foundation does backup.
For issues and PRs, I don't know for sure.