But if all Uhura did was run and repair the space radio, then why did they ever bring her on away missions?
I don't believe anyone ever specifically said why. Are we speculating? Well, then I could speculate any number of reasons, none of which need have anything to do with her being any kind of linquist.
There is simply no evidence in TOS of her being any kind of language wizard - and at least one scene in the movies that shows she is not