I must bend knee to Tolkien to even speak on this subject. At the heart of fantasy writing, you will always find his work. Not only did he break ground on the genre in many ways, he also started a long-lived trend of creating new languages for fantasy races. Unlike many modern examples, Tolkien had the knowledge to back up his language creation. That gives the rest of us a hard standard to live up to, but one I do not begrudge him.
Whenever you introduce new races or talking species within a novel, it makes sense that they might speak another language. There are many ways to dance around this issue. Probably the most widespread method is to create a “common tongue” that all the races know for trade reasons or the like. That seems reasonable enough, but what do you do when you dump your character into the midst of a different race of people who have no real desire to accommodate by using that common tongue?
In books two and three of the trilogy I am editing (again), I do just that and she must try to pick up enough of their language to get by. She has one mentor who is willing to speak the trade tongue part of the time, which helps with the learning process, but it is still a challenge.
What did I know of foreign language when I started this? A little bit of Spanish learned in High school when I really wasn’t interested. I’m certainly no Tolkien. About the time I was working on the second book of the trilogy, I started learning Japanese for a number of completely unrelated reasons, not realizing how it might benefit my writing. As a supplement to learning the language, I have also put some effort into learning about the culture it comes from. What I have found through this process is that it gave me a valuable insight into what my main character would face trying to absorb a new language and culture.
If you write fantasy (and any other genre in some cases), I think learning at least one language and culture can prove to be invaluable research. Learning Japanese has been very useful because of how it is structured in comparison to English and that it has some interesting history behind it, such as the development of their written language. I’ve come up with many ideas on how to structure the languages in my book and to show the struggle my character faces. In some ways, I almost envy her being submerged in their culture and forced to learn quickly. Learning a language is hard when you have no one to practice on.
If you can make the time, even if you don’t write, I recommend learning a new language. It is a good experience and you can do a fair job on your own using programs like RosettaStone along with some additional supplementary resources. Pick a language you are interested and have fun with it. The benefit to those of you who write will just be a bonus.