It also makes adaptability more necessary in cases of people moving between regions of countries with large linguistic differences, replicating the culture shock that would happen in real life if someone were to move from somewhere that speaks Mandarin Chinese to somewhere that speaks Hakka Chinese.
Statistics from Ethnologue were used to compile a list of every language actively used today. The only languages not included are the ones formally considered extinct, dormant, nearly extinct, or moribund. This leaves us with an increase in languages from 121 to 6071.
Alongside the massive increase in languages, I have increased statistical accuracy of second languages significantly, with an increase in knowledge of English in much of Europe, Russian in Eastern Europe, various second languages throughout the world, and a significant decrease in knowledge of Spanish in most of the United States.
Cities where the majority language is different from that of the country as a whole are acknowledged, and countries that don't have a singular national language (Papua New Guinea has 839 different languages for example) will be les culturally homogenous and therefore have significant differences in feel depending on where in the country you choose to manage.
Discussion: FM21 Full Language Update
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Southern Min would probably be a more apt comparison, though the only place in the Chinese speaking world where you'd find high rates of Southern Min (Hokkien) fluency and low Mandarin fluency would be in southern Taiwan. Even there, a theoretical team in, say, Tainan would not likely teach its foreign import players Taiwanese Hokkien instead of standard Mandarin.
The rest of the world may be different; however, in the Chinese speaking world, this idea really needs some work. You're not going to see a lot of foreign players learn Korean if they start to play for Yanji City, for example: they are much more likely to learn standard Chinese first.