I've read that Charles XIV John of Sweden never learned the Swedish or Norwegian language while ruling over his people, so did he have a translator come with him whenever he delegated affairs. I know that some of the aristocracy spoke French, but surely not all of them did and some sort of translation would be required?
Actually, it's probably more likely than not that they would know french. It had been the lingua franca of the Swedish (and most European) aristocracy for the better part of the 18th century. Gustav III is known for having been particularly francophilic and french was the language of his court. (Even after being shot, he exclaimed Je suis blessé! rather than something in his native language) Most French loanwords in Swedish (and Norwegian) date from the period around 1800 (paraply for instance).
I haven't read that Charles XIV had a dedicated interpreter. Per the above it's unlikely he would've needed one. It's known that [Count Brahe](http://en.wikipedia.org/wiki/Magnus_Brahe_(1790%E2%80%931844%29) and Charles' son Oscar often interpreted for him.