Sounds like it's due to Latinization of the names. In Latin the ending -i can mean plural subjects (nominative plural), as in Germans, or singular possessive (genetive singular), as in of Germany/German's. I'm unsure which is the actual case used, but either meaning works.

You might want to try r/linguistics there is probably a linguistic reason. My guess it was either latinization in using the plural. It doesn't look like a Germanic feature to me.