G'day again,Wilfried: I'm sure the oldest of the German names are (now) German words ... and used by the local people for millenia. But ... I still cleave to the notion that no people has need of a descriptive name until they are confronted by other peoples ... and such descriptions usually come from the people coming in, not the settled ones who have no problem knowing who they are. I also suspect that two or three thousand years allows a lot of space to create endurung "folk myths" about the meaning and derivation.
HuwG: I'm sure you are right about the way the Welsh use Sais ... and it is part of the same process of name-giving (~-calling?) and analogous to similar epithets used by the other Celts of Britain. (Even if the "Saxon" name is somewhat suspicious ... we were all taught that England was taken over, post-Roman Occupation, by "Angles, Jutes and Saxons" - but the linguistic evidence suggests that everyone (well, the Venerable Bede, anyway) forgot to mention the Frisians!
BTW: The risks of accepting (what you think is) the local, native, name are beautifully illustrated by an Australian example. Down in Melbourne, (Victoria, capital of our south-eastern mainland state) they have an annual festival called Moomba: because someone thought that this was the local native language's word for celebration, or something similar.
Lately they have been able to check with source documents and/or native speakers ... and have found the the name of their festival means: Buttocks! Maybe the native enquired of was giving a succinct message to the invader (aka "colonist").
Regard(les)s,
Bob Bolton