The masterminds behind Dungeons & Dragons had a bold approach to languages: every race has their own. Boom. Simple, straight forward, and upon reflection, idiotic. The world in which Slothjemia exists has a much more nuanced approach, and is as delightfully complicated as real life.
Racial languages are only rarely used, and only then when the race in question is sufficiently isolated to warrant there not being any significant variations. One of the exceptions is the Drow elven community. Stuck deep in the underdark, and separated by many millennia from their elven kin, the language of the dark elves is as distinctive as it is widespread (in the underdark). Since Drow elves are not found on every continent in Slothjemia’s world, those that do exist are in some proximity of other (albeit rival) dark elves in the underdark. This makes it close to English in the real world; spoken in many different places, but with regional dialects to spice things up.
Unfortunately, this puts them in a place wherein surface elves and the dark elves do not share a common language. Surface people are much more likely to utilize national, or regional languages. These surface languages often incorporate elements of “racial” speech patterns. As has already been discussed, Slothjemian is a hybrid mix of goblinoid dialects and Geldenreich common. Similarly, Fanolanian is a mix of elven dialects and human languages that were prevalent to the area. Speakers of Fanolanian would be able to talk to a number of their neighbors with little or no hindrance, because the surface world mixes and melds much more readily than do the races of the underdark. One of the only traits shared by every single group of people in the underdark is xenophobia. This feature is hardly conducive to linguistic mingling.
There is, however, a wildcard; ancient languages. On every continent there is a terribly old, and rarely utilized means of communication. For example in the real world, consider Latin. Not a lot of folks speak Latin these days. It is the original form of many modern languages, and if a person really put their mind to it, they might be able to make out some words or phrases. It might be easier to wrestle the Latin intent if you come at it from Spanish or Italian, and more difficult from French, but that is hardly a rule. The point is that ancient languages in Slothjemia’s world tend to be the domain of magic users or priestly types, and that transcends surface and underdark issues. If a Drow elf is learned enough to have mastered the ancient language of the surface world above them, possibly due to the ancient language being a vital element of basic magical incantations, then they would be able to communicate in that tongue with a surface dweller who also has an understanding of this language.
This is just a long way of explaining why nobody ever goes anywhere.