I think this question was raised before and the answer is, they needed exotic-sounding names for tribal people and landmarks of Rimworld, and picked a source on a whim. This list on Wikipedia appears to match almost exactly:
https://en.wikipedia.org/wiki/List_of_Galician_words_of_Celtic_origin
https://en.wikipedia.org/wiki/List_of_Galician_words_of_Celtic_origin