We did a merge this morning for XWiki 10.8 so depends if your changes were before or after that ;-
More generally they are merged on the fly when they happen and are thus present in the next release.
Not really. Anyone can review but for German I don’t think anyone from the xwiki dev team speaks it. We do cursory review for the languages we know but apart from this it’s a community-based review, as a best effort. We don’t block if a translation is not reviewed.
One important constraint was that XWiki has a huge number of translations which was exploding the quotas of all the online services we looked at so we needed something we could install.
I’ve personally suggested to xwiki team the use of Weblate to manage translations.
I do use a lot of other online services for translations (transifex, crowdin, getLocalization, …).
I can not speak from a technical point of view (setup, volume limitation, performances, integration with a Version Control System, cross authentification …) but from a user perspective it’s really a great tool. You can contact me if you want more details on Weblate benefits from one of its “free” users ;-).