Languages are a convention – speakers create them by using them. So if enough English-speaking people adopt “Mongolian” as a (external) demonym over say “Mongolien” the former becomes the accepted word. English is somewhat specific in that out of major world languages it doesn’t have a formal regulatory body that would be the authority on standardization so it’s even more flexible than some other languages but in general, all languages depend on the common usage among the speakers.
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