I was wondering if it would be possible to add separate soundtrack for disc which have multiple soundtracks in the same language.

Like French (normal and Quebec), Spanish (South American and castellano) and Portugese (Brazil and normal). Strangly for Dutch there is a second option called Flemmish. As Flemmish isn’t a separate language I was wondering why these other common variants aren’t separate listings.

You already do for one dialect ;)

But isn't it then correct to remove Flemish and change it Dutch. All my Flemish movies mention language as Dutch, only animated in my collection use the term Flemish and often as Dutch - Flemish. That keep it clear.