What the heck, I'm going to start making up a language - partly as an exercise in linguistics, which I've always been interested in but never properly studied, and partly because it's one of those things that the fantasy authors I really admire have done - and if JRR Tolkien, MAR Barker and Jack Vance can do it, why can't I? (To those of you who might be reading this and thinking: this man has too much time on his hands...well, it helps that I don't watch any TV.)
The language I'm going to make up is the Yellow City Trade Tongue, a lingua franca spoken in and around the Yellow City and the places up and down the coast with which it trades. It's a simple language, as most lingua francas are (this also helps me start small), rather like Chinook Jargon or Sabir. What's unique about it is that it's the only language which both slug men and human beings can use to communicate properly with one another. Slug men's simple mouths, which lack teeth and have only a rudimentary tongue, can't produce many of the sounds which humans use in their languages, and humans can't produce the pheremones which slug men use to augment their own speech. Yellow City Trade Tongue provides a way around that.
Currently, the only thing I've really decided on about Yellow City Trade Tongue is that it is consonant-poor. The labiodental, dental and alveolar consonants are nonexistent, as are the nasals - slug men don't have bones or noses. This means that the only consonants available are most of the plosives, some fricatives and approximants, and the bilabial trill. (You can follow along with what this all means with the help of this handy IPA chart with sound samples.) By contrast, it is very vowel-rich. Almost all of the vowels are present, as well as dipthongs - slug people have no problem producing those sounds and in fact could probably create more.
I'll be going away and thinking about this some more tonight, as well as thinking about some fundamentals of grammar.