Argos has been continuously inhabited for the past 7,000 years, its recognized as one of the worlds oldest cities. How did Solitude existed back then? Because Skyrim was the first province inhabited by man, and if five thousand year old cities can exist in real life they can certainly existing in a video game. Serana wasn't conscious during her imprisonment so speaking really isn't a problem.
Magic
Fair enough on the cities, but it is impossible that a language would not change over thousands of years. Hell you can observe changes during your own lifetime.
I'm pretty sure there's a trope for all this linguistic nonsense.
In any case, unless the work is made by a linguist, language tends to get taken for granted in just about every medium. So you're best off just ignoring those kind of details.
