Does PL are all from the same "real-nation"? No.
did user didn't know buzzerbeater before searching a PL, and there are many national PL too. Where people look for playing against people with there nativ languages.
You are "going around the bush".
This is not what will make users go from this game.
Doesn't Warcraft (for example) allow local users only, on a single game? No, it is not.
does BB allow people from different nations to join? Yes.
I didn't play warcraft, but i know that one buddy of me was in a german guild and that isn't rare. But there you can jump from one to another till you find the dudes you like to play with, do you want the same here. Play half a season in this league and switch to then into another one? That wouldn't working, even after season ending.(and even then you couldn't handselect your competitors or enviroment like in warcraft)
Still "going aound the bush", and still not facing the fact that my point proved - that the real-nationality is not what mainly affect the groups that are part of a single game / league / etc.
Does all those leagues on smaller real-nation will grow upon that? Yes they will
but they would be still ugly to play, especially if you are the only dude in thailand who actually speaks thai and just that.
Not really. The amount of conversation inside a league is very limited at best.
Thai users, will still have a thai forum (instead of Thai-BB-nation league, or something like that, which currently exists).
The work for the NT coach may be a little bit harder due to conversation, although probably will get assisted translating by other users and such, but will have much better variety of NT players to chose from for that NT team.