Yes, we can all agree that language evolves. However none of us know how many of the forms we hear today will become the standard and how many will simply be the fashion of the 2020s and disappear again.
Teaching any language to foreign students, ,most teachers tend to stick to a more "correct" language than they might use when just chatting to friends.
No-one who has learned their English in Amsterdam, Copenhagen, Vienna of Cologne will have the slightest chance of understanding "Come oan, get aff, hen" or anything else said in broad Glesga'
My DH, a non-native English speaker couldn't fathom what a taxi driver from Paisley was asking us at Glasgow Airport - which really made me wonder how he would cope in Glasgow.
Short answer - he didn't - I having been born and bred outside Paisley did all the talking and I still do not fathom what was so incomprehensible about my conversation with the taxi driver, as Scots, as spoken in Paisley, is nothing like Glaswegian and far, far easier to understand, being nearer to RP.
My sister reduced herself and me to giggles by saying, "Shall we have a wee cae-ik wi' oor coffee?" and even that had to be translated for him to "Shall we have a little cake with our coffee?"