Defining languages are hard. People have tried things like “if two people can communicate they speak the same language” but there are just too many cases where this does not work. So instead we have kind of given up and instead define languages similar to nations, and often very related. Basically go around and ask people what nations they belong to and what languages they speak and you have your list of both nations and languages. So you end up with a Scotland with its own language and nation separate from the other languages and nations on the British Isles.
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