
I sort of picture it as there being one overarching "Common" tongue, as well as individual languages that may only have limited usage nowadays. Meaning, you might grow up in Ul'dah, but since the Common tongue is the most used, you really might not know whatever the native tongue is there. However, if your family is super traditional and stresses history and sticking to your roots and all that, you may have grown up knowing the native tongue... even if it doesn't have much practical application in every day life.
On top of that, I wouldn't be surprised if the more isolated people, especially Miqo'te tribes, had their own language and were only semi-familiar with the Common tongue. I figure all this leaves a lot of wiggle room for RPers to have a variety of accents and dialects and also allows some to RP characters that don't always understand the language. Makes for some amusement when a character takes you literally on some slang!
On top of that, I wouldn't be surprised if the more isolated people, especially Miqo'te tribes, had their own language and were only semi-familiar with the Common tongue. I figure all this leaves a lot of wiggle room for RPers to have a variety of accents and dialects and also allows some to RP characters that don't always understand the language. Makes for some amusement when a character takes you literally on some slang!