Ok, so I have started the worldbuilding process by creating several different communities and giving them each their own personalities. I ran into a problem, though, when I created a desert dwelling community that doesn't identify with the rest of them. While all of the others could be considered portions of the same thing, this one, due to its history, demands to be separate.It's separate not only metaphorically, but probably physically, by maybe a mountain range (haven't made much of a map). In a situation like this, should I make a second language that this desert dwelling community speaks, or should I make a similar language, or just give it the same language as the rest of my world?
It depends on the interactions they have with each other, the more tied they are the more likely the language would stay the same, the less, less. China has so many dialects, sometimes people on different sides of the same mountain can't understand each other.
Languages form by populations splitting, forming accents, that become more and more extreme, until they are mutually unintelligible. Your split communities would do the same thing, unless both versions of the language are artificially kept in stasis by the speakers. Pedantic teachers with rigorous pronunciation and grammatical rules could keep a language the same forever.
The creation of a new language does take hundreds of years though, so if the isolation is recent (relatively), you could get away with thick accents and a few words that need translation.
If you want people to be able to talk to each other, but want some realism in handling the separation between the two societies, you can have them speak and/or write in different dialects of the same language. That will let the characters communicate with each other, but still introduce a level of communication difficulty that one would realistically expect in those situations.
(This is an offshoot of @Fungo's answer about "Pedantic teachers with rigorous pronunciation and grammatical rules" maintaining a language.)
The desert people could have a collection of sound recordings (or videos) that they listen to regularly, across generations. That would likely keep the pronunciation from varying too much. One type of recording that would probably be listened to over and over is recordings of religious events such as miracles, sermons by holy figures, or readings from sacred texts.
Give it a differed language! Living separately means living with things that need new or different names, and words that have little meaning, (Ocean, snow, mushroom, things that aren't in deserts,) not sticking around. They have a totally different set of things that 'matter,' and thus, things to be talked about.