Each world in Dark Mirror inhabits it's own race, language and culture. Depending on what plot you are playing the gods may (or many not) have given players the ability to understand all languages. This is a description of the races, cultures, and the languages they speak.
As this is a game designed for humans to play, I'm going to cut this short. lazy GM is lazy. Think of a medieval Bavarian countryside. That's pretty much what we have here. The language is pretty much German (however, whatever language the game is conducted in is the "Universal Language" of the game) with a few words added in for things that don't already have words (due to not existing in the real world, or popular fantasy culture).
The Aeron are a race of anthropomorphic bird like people. A firm class system has been established among them. However, things are a bit tense as every class feels it is somehow above the others. The so-called upper crust feel they are better because they have more money, and thus spend more on intellectual and artistic pursuits. The Merchant class feel they are superior, as they travel the world and experience more culture (and not the same culture the high-class people think they get when they "Travel Abroad").
The Artisans and Tradesmen feel they are superior as they are working to master something. As it is only one thing, they have more time to spend with family and friends and be social. The peasants feel they are superior because they don't need validation of a skill, or money or culture. The have what little they have and they share it and are happy. Each class simply suspects the others are only lying to keep face, and do not actually feel superior. This is also not to say that everyone in every class subscribes to the ideas.
Although all adult Aeron can fly, Airbuses are provided on a regular schedule. This is because both the young and elderly Aeron have difficulty flying for long distances, Some feel it is dangerous, and other simply feel is is undignified. There is a popular saying among the Aeron-- "If the good lord had wanted us to fly, he would have provided us with little bags of nuts.".
The language uses a syllabary (similar to Japanese) but is very Germanic as far as grammar structures are concerned. It uses various chirps and tweets. It is written in a way simmilar to music.
The Fwissh live in a city of vice, crime and algae. The organized crime group, who once had control of the city is now fighting against a popular street gang for control. Caught in a gang war, many Fwissh from out of town still flock here for the nightlife and casinos.
If you have a business, you have an allegiance. If you have kids, you have an allegiance. Normally things don't get too bloody in the more residential areas, but after dark, you;d better carry a gun.
The language involves a lot of stops, and is written with a lot of loopy letters. It uses an Alphabet.
Tribal people who talk little and just do, they believe that actions speak louder than words. Actual relations within a tribe a often left ambiguous, as everyone is Kinfolk.
The language is mostly written, and pictographical, with some ideographics thrown in.
Spider people. They live in cities which are built entirely out of silk. They are very religous, herbavores and frequently take time to worship their gods.
Somehow, in a world with no living, natural metals and gemstones have manifested themselves as beings. Always looking for a good party, they never stop.
Nuclear bombs and flower petals. What's not to love? Although these plants might seem nice on the outside, inside they will stop at nothing to control the world. Advanced military technology, this race could be a thorn in your side.
Cloud people who just don't give a damn. It's one-for-all and all-for-me with them.