Cath-Oswic is anywhere from 400-600 years old. He's going by how old his favorite swords are. The Claymore(Celtic) and Zweihänder(Germanic). He was born a vampire and was in an arena for a long time, so he didn't actually talk all that much. So everything about his voice is a bit odd. He was about ~16 in the arena, and was in there until ~16-20 years ago, so... Yeah.
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Tauekel was born in the 1300s as well, but he lived on the Russian northwest borders for a long time. Like, centuries. He had a whole different mafia there and everything. He came to America probably in the 1800s or 1900s, and yeah. He never really lost his accent too much since he very much values his culture and definitely still makes sure to find a way to keep it.
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Shanahan doesn't really have any accent.