The language he speaks is English. His birth was in Birmingham thus British nationality. However, his accent is a strong Welsh thus I would consider him the Big Welsh Bomber. Some could argue his accent was Yorkshire, and when you think about it Welsh and Yorkshire accents are very similar, but anciently the Welsh first settled in York so that explains it. They also anciently settled in Cornwall hence the similar accent affinity there too. Basically to sum it up he was born in Britain and spoke with the Welsh accent/dialect, and as the ancient British were actually Welsh, you should say he was Welsh.