Not really. The existence of a unified German nation-state is less than two centuries old. Sure, there was a place called "Germania" by the Romans but it was just a name for a place and the categorisation of all Germanic tribes as essentially one barbaric people was just racism on the part of the Romans.
Just like even though the word "America" is twice as old as the United States, and just because that label was adopted by that country doesn't mean the country is as old as the label.
The Holy Roman Empire was not a contemporary of Julius Caesar nor was it united. If your subdivisions are fighting wars with each other then I don't consider that a proper "country".
I think I’m taking this too seriously for historymemes, but if someone from the future came to tell me something was invented by “Slavia”, I’d probably assume it was a country northeast of what I know as Austria, even though those countries are currently at war with each other. I can imagine a future in thousands of years where they’re politically and culturally united, even though I can’t imagine one in twenty years.
Don’t go bringing that crackpot Edward Gettier into this. Caesar would know the Germans (those who hail from Germania) as a disorganized, unorganized group of tribes with a common heritage. That’s a justified true belief, or at least as justified and true as one could expect of him. His beliefs would not cover a unified German nation, at which point there can’t be a belief part of a JTB.