I don't think saying that is "sometimes" included or "widely considered" to be is useful, because it simply is included within the category (and though it's true that Wales and Scotland have distinct devolved status in the UK, Cornwall does not). Of course there are no doubt people who think the whole concept of "Celtic nations" is nonsense. Fine. But the concept exists and Brittany is part of it. I go with a shortened version of Ultima's line: "Due to its Celtic language, Britanny is regarded as one of the sic Celtic nation".