Taldûnian is, as far as anyone knows, a language isolate. There probably was a mother language and at least some sister languages, and there are hints of similar languages being spoken in the areas around the original barony, but its bloody and oppressive history has wiped out any traces. By the time the empire started to take form, there was only one language left.
Of course, the story after the beginnings of the empire is another story.
In the capital, the original form of Taldûnian is still spoken. There is some new vocabulary and such things, but overall it is the same. Perhaps the old age of the emperor and his generals (long story short: extremely powerful sorcerers) has had a very preservative effect.
The capital version of the language is still used in formal communication throughout the empire. However, as the empire is so large, there are now daughter languages spoken in different areas – eight variants are formally acknowledged, though this is of course disputed. However, not everyone considers these true languages – the old debate of the boundary between language and dialect is alive even here. Of course, as everyone in an official position needs to know the language of the capital, the differences are not as great as they might have been.