Hello so we often hear tribal names from the Romanized side, but I was wondering how the peoples of the various Celtic, Brythonic, and Germanic tribes called the Romans. Not necessarily an exhaustive list but major terms or root words that are shared with some derivation based on the specific language and tribe. Do we have any record of this or as the Romans were the conquers in some cases erase other names?
We actually have a good deal of the Gothic language attested. Most of the corpus is available here for you to look through: http://www.wulfila.be
The Gothic word for Rome is attest in the dative case as Rumai, so it likely would have been Ruma in the nominative.
We have the word for "Romans", meaning people from Rome, attested in the dative plural as Rumonim. This would be Rumoneis in the nominative plural. We don't have it attested in the singular, but because it was likely borrowed from Latin Romanus, it may have been Rumonus. This would give us a Gothic u-stem in the singular and an i-stem in the plural.
The adjective "Roman" is not attested at all, but would probably have been Rumonisks.
There is a small amount of Gaulish attested, but not enough to fully reconstruct the language (though some have tried). I'm not aware of any texts that mention the Romans though.
Later, we see speakers of Germanic languages referring to inhabitants of romanized regions as some varint of the Proto-Germanic *walhaz. This is where we get words like Welsh, Walloons, and Wallachians.