“A language is a dialect with an army and a navy.”
I don’t know who (if anyone) is credited with first coining that one (braces self for a barrage of information), but it’s a venerable and very true observation. For example, Portuguese and Gallego (Galician) are very similar, but one has had unquestioned status as a paid-up legit ‘language’ and the other has only in the last decade or so fought its way up from ‘banned dialect’ status. If the Spanish state had managed to hang on to Portugal from 1640 until now, Portuguese and Gallego might not even be considered as separate from each other, let alone as ‘proper’ languages.
And in the 16th century Scots was the language of literature, scholarship and government in Scotland; if James VI hadn’t acquired the English throne, it might still be.