Might even be possible
I was going to suggest something along these lines as well, however I think you would wind up with a language closer to a Flemish / dutch at the end of the day. but I could see it if the empire could be held together an enlarged Holy Roman Empire.
Only issue is that in the good old days such empires usually had several languages. Just look at Austria-Hungary for starters. YugoSlavia, even the Russian Empire who were hell bent on trying to get the populations to speak Russian, never could kill Lithuanian, Latvian, Estonian, finish, polish, Belarusian, ukrainain, Georgian, Armenian, aziri and other assorted languages.
so you just may wind up with a big empire that the further east you go the more German they speak.. the more west the more gaulish with a a large central group speaking some form of Flemish/dutch hybrid of the two