to clarify a misunderstood point...

the germans and celts had a writing system, we just have almost no record of it. so, you hear christians say things like "the germans left us without a history", but this is a deeply misleading statement. what seems to be more true is that the romans completely eradicated any trace of their writing in the areas they conquered (much as they did with the carthaginians), leaving only scattered remnants in areas that never saw direct roman control, like the ogham scripts in ireland, the norse sagas in iceland and the gothic runes in the east.

so, it's true that virtually everything we know about the celts & germans was written by the romans, but it is actually rather dubious that it's because nothing was written - and more likely because their own history, as they wrote it, was eradicated as an intentional act of genocide.