Language effects how people think. It shapes our thoughts, since they are expressed in language.
The following is just my own idea on this:We know that there is a translation spell in felarya. There's no details on how it works though.
As far as I know I'm the only one who has attempted an explanation. So, the spell is slightly telepathic in that it gleans the meaning of the speaker and passes it too the listener. The listener still hears the words in a different language, but understands the meaning.
However, a requirement of this is that the listener's frame of reference and experience is similar enough to the speaker to allow the meaning to be accurately transferred. What if someone says the word "wheel" and the listener has no concept of spherical or cylindrical shapes? The spell would fail because at that point translation is impossible.
Of course, someone can suggest that the spell also passes the concept behind the meaning too, but that is very dangerous. If a completely alien sentient life form appeared and started speaking, the amount of information being forced into the listener's brains would send them insane.
Now that's established, I can get to the relevant part.
Because of the aforementioned effect, language in Felarya and especially Negav would be amazing. Since the meaning behind each word is easily understood (within limits), people would be learning new words all the time, the collective vocabulary of the city would be enormous. I'd say almost any time two strangers interact at length, new words would be learnt from the exchange. Of course, many of these words would be interchangeable. I'd say one way to tell where someone is from in Felarya is by what words they use for normal speech and slang, since new words may be used for something slightly different than their original meaning.
On top of that, due to the translation effect, the meaning of the word would be coloured by the speaker, since people from a certain area tend to think in a certain way, the meaning of the words they use (and especially slang) would be effected by their thoughts and way of thinking.
So there's two levels of accent and dialect here.
(Assuming of course people decide to go with my idea).