For those interested in a detailed modelling of languages in their games, I can recommend a couple of non-Runequest books.
The Power of Babel is a good nonfiction book about different languages, and the theories of how they evolve. One point made in this book is that technically there are not different languages, but only different dialects.
The Iron Kingdoms Campaign Guide is a D&D setting sourcebook (boo, hiss on D&D

) which is very detailed. The section on languages provides a very well thought out example of how languages can be implemented with a great deal more detail than is commonly provided in a setting.
For myself, one of the things I've been doing is frequently writing down just a couple of notes about languages that are used in a setting. For example, in my homebrew, I note that the Kosaka language has a number of instances where words sound similar but mean slightly distinct things (similar to "lie down" and "lay down" in English). I sometimes make up just a few words to give a feel for the language ("tritiana" (woman), "tritiyana" (beautiful woman) in Kosaka, for example).
Some notes I've made for languages in different settings:
Nomofra -- this language has a sacred alphabet and more pronouns than anybody knows what to do with. Each word and name is considered to have a sacred meaning, based on the letters that make it up. (Hebrew is a little bit like this, with the Hebrew alphabet being the basis of the Cabalah.)
Draconic -- is actually two languages. The language spoken by dragons is pretty much impossible for humans to pronounce, so there is a form that has been developed that most races can at least approximate.
Kosaka -- has different words for siblings, depending on whether they are children of the same mother, children of the same father, or playmates from the same clan. Has different pronouns for the feminine gender, where human and closely related humanoid females are referred to with different pronouns than those used for animals, and those used for other humanoid and sentient races. Different inflections also indicate age, status, and in the case of animals and nonhumans, "kind".