I do know that it will take a grand amount of research and effort on my part, but I thought I'd tap into the well here for some insight. Basically, the original language of the humanoid race in a fantasy product of mine is based on Latin (in the most recent form before it became a dead language; it was 'seeded' to them by the deity that created them). How would such a language naturally evolve due to certain constraints, such as the evolving religious mythology (wherein the triune 'godhead', as it were, is a mother, father, and child)? Or interactions with the languages of other species (the bellows and grunts of the orc archetype's symbolic language, or the sea-dwelling race's musical language, or the impressions by which various spirits communicate with the living). Ideas and guidance on how to study the evolution and creation of a language is what I'm looking for.
If I were you, I'd study the evolution of the English language. English has been influenced, modified, torn apart and reassembled by everyone from the Romans to the Vikings to the Normans, etc. etc. etc. The development of English has also been very well documented. If you want to know how languages develop, English should be your primary case study.
Personally, I wouldn't put too much effort into this. Only work on those words and phrases that will be a part of your book, like honorifics, greetings, etc. Otherwise, just describe the language, like: "The Orcs spoke to each other in their clipped, rhythmic language," or, "The spirits spoke to each other, but in nothing that sounded like language. It was a communication of ideas and concepts--the spirits didn't have tongues or mouths or breath--which Tanner's human brain translated into a series of airy, quavering sounds." It really interrupts the experience of reading when you have to keep flipping to the back to translate dialogue, and readers will only put up with so much before they get bored and move on to something else.
It's more so that I can get a concept of consistency and natural fluidity across. While the narration will be in English, as would the dialogue - albeit in different dialects to reflect different offshoots of the original language - I'm going to leave some untranslated bits in (like titles, names, events, and other important entities). I just want the framework to be solid, so to speak.
I can't help with the specific request, unfortunately, but inline with what ManOrAstroMan said I wouldn't try to get too involved with it. The influences on evolution of languages are very wide ranging and variable, to the point that it'll probably give you a big headache trying to take it all into account. There might be some overall guidelines you could grasp, I would try doing Google searches to try and lock down whatever (if any) those might be. A bit OT as it's in an unrelated area, but illustrative of the problems of a writer trying to get to grips with known 'science', I've been trying to understand the way moons, planets and stars orbit and influence each other, particularly with regards to co-orbits and orbital resonance. All I can say is it's a minefield, I now have a headache, and I still don't understand enough to know if what I want to do is actually possible, but I get the astronomical info just enough to think that it's possible but highly improbable. I've had to settle for not doing more than that, because much as I would like to know precisely where any given moon or planet would be so that my characters (more than one of which are astronomers) can look up at them and 'see' an accurate sky, I've come to realize that to get the motions right in accordance with known laws (Kepler, Newton, Lagrange, etc) would be seriously beyond me. So I will have to approximate the sky and hope for the best that readers who are knowledgeable of such things won't be too picky with me.
