Much like Earth the world this story is set in has many languages, and different dialects and variants within those languages. To further keep people on their toes, most species/races have more than one language. While the language of the Avali is the closest thing to a "common" tongue there is, the language an aval from the far eastern parts of their empire speaks is quite different from the aval living along the northern Doelm Isthmus, i.e. close to humans. Several of the human societies that once had a distinct tongue of their own have over time merged their language with that of the neighboring avali. This merging of tongues has formed a sort of a hybrid tongue that's spoken, by both human and aval, where the two cultures live in proximity to each other.
One language that is consistent across the species is that of the ferenabu. This is fortunate for everyone else, as one chirring noise can sound much like another to human ears.
This complex linguistic landscape is why, for the sake of clarity, we are not using a different font for every language. Where appropriate and where it doesn't mangle the story, though, we'll try to make it clear that folks aren't speaking the same language.
The languages of the Thrall and Ferenabu are in good part unpronounceable by the other species, because of the great differences in physiology. Conversely, a thrall or a ferenabu can't make certain sounds from those other languages. So, in general, the tendency is for each species to learn to understand the other's language, but to continue to speak in their own tongue. Think Chewbacca talking with Han Solo.
A ferenabu can pronounce few words of the avali speech if absolutely needed, but the limited vocabulary available to such a different physiology does not lend itself to an in-depth conversation.