80% is pretty unrealistic but I see your point - especially in the cities it might get that high. I'm not certain this itself leads to language change as much as it might seem though, for a few reasons:

• Multiculturalism really isn't that successful generally and ethnicities/cultures tend to conglomerate together in their own little enclaves (This is why you get entire regions in the UK that can't speak any English at all nowadays).

• As an immigrant of a sort myself surrounded by Asians, I tend to double down on my English identity. My grammar has improved far more than it otherwise would have, and I think this works the other way around too.

• When pushed to use borrowed words from, say, Turkey, I'd assume Germans by nature would push back and emphasize proper German in the same way I do for English. Even kids who get influenced by accents and such tend to revert back to whatever their parents are speaking long term.

All that being said, European languages are by default a chaotic soup of random languages throughout history, so I'm sure some stuff will stick (Like how I mentioned English infiltrating Japanese - even more pervasive in Korean)