Not sure about the handling of Germanic languages. It seems to me they'd survive better on the periphery, like in Britain and Scandinavia, rather than in the heart of the continent, much as the Celtic languages survived in insular environments IRL. [...]
Perhaps you understood that I was going to have one Germanic language, because of what I wrote above? In this alter-hist-world Germanic languages are spoken pretty much in the same areas we know, basically (and grosso modo) central Europe, southern Scandinavia and England.
German as spoken in central Europe won't be very different from Middle High German (c. 1050 - 1350), so there's no problem here.
The big difference would be the tongues in the Germanic British Isles, since there's no Norman invasion and some Celtic languages (what I call "Scottish Galeic" in the notes above) are heavily influenced by Norwegian dialects.
Edit: I'm going to post an ethnolinguistic and political map of Europe I've been working, and it will be clear the extension of the languages I mentioned.