It is implied in some of the developer posts in those Lore forums that all racial languages have faded into obscurity or disuse (except for Roegadyn, who still use their language but -only- for naming their children).
What I'm thinking is that all races have lived together in Eorzea for so long that the languages fell into disuse or merged together into the modern language. This would have taken many centuries, mind, but Eorzea has a lot of eras in the calendar. It's not hard to assume that our nomadic tribe adopted the modern language so long ago as to make the old Miqo'te one meaningless.
What I'm thinking is that all races have lived together in Eorzea for so long that the languages fell into disuse or merged together into the modern language. This would have taken many centuries, mind, but Eorzea has a lot of eras in the calendar. It's not hard to assume that our nomadic tribe adopted the modern language so long ago as to make the old Miqo'te one meaningless.