This post is in answer to Meg's question about using two non-English languages in a story - thanks for the question, Meg!
First, some general orientation. We're not talking in this case about using world languages, or speakers of existing world languages, in a story full of English speakers. That would be an entirely different issue. This is a case of two fantasy populations, each of which has its own language.
When fantasy populations have their own languages, this means that no readers have any hope of finding a dictionary to allow them to understand the languages the author has created. I have done this before, and I generally referred to the difficulty with using such languages as "the translation problem."
Question 1. How much do you render in the language, and how much in English?
I've addressed this question before, but the basic metric is this: the more of your created language you use, the more alienated your reader will feel. So if you're working with an Earth traveler among aliens, and you want to emphasize that person's sense of alienation and confusion, use more of the alien language. But if you're working with a point of view inside a population of speakers of the created language, then use more English translations of the words in question, and only a minimum of the created language's vocabulary.
Think about it this way. You are teaching your reader every vocabulary word you choose to use. This requires reader effort, and you don't want to overload them so much that they can't follow the story.
So here we are. In Meg's case, she's working with the point of view of a speaker of a fantasy language.
Question 2. How do you deal with the internal pov of a person who is not an English speaker?
I'll call this the one-language problem. Assume that everything that occurred in her native environment happened in her native language, and its cultural context. She's multilingual, but for the sake of simplicity I'll assume that her thought patterns were primarily influenced by her native language. The way you write the English for her will not sound like the English you would write for a native speaker of modern English (obviously). It should use the metaphors, concepts, manners, and cultural sensitivities of a native speaker of the language she speaks. I call it the translation problem because you want it to look something like a translation of her native language into English, rather than just English.
Now, when I work with alien languages in my short stories, I take the "translation" further, and try to make alterations in the structure of the English I use, to reflect the use of the alien language. This requires a sense of what the structure of the created language is, and how that structure might influence the speaker's use of English - but I don't necessarily recommend it in this case. You're working on the inside, in a fantasy context where the main character should probably be considered "home base" for the reader. So I think that any significant grammatical alterations would probably be too distracting.
So in Meg's case, the first step I'd probably take is working out how her main character's speech would come across based on her native language. Forget for a minute about the second language issue, because if you can figure out how her native language will influence her English, that's going to be roughly what you need to discover how her native language influences her use of the other language. Especially since both will be written in English anyway.
Now, if you're working with a second language that the protagonist doesn't understand, you can simply have them not understand it, or partially understand it. That is, if you're using a close point of view (either first person or close third person).
But Meg has a protagonist who understands and speaks the second language. So that leads us to....
Question 3. What do you do when you have a second non-English language?
Once you've established your "base" for Language 1, start looking at the other language. Pick an antagonist (because in Meg's case, the second language is the antagonist's language) and go through the one-language process again, for the other language. What concepts, metaphors and other features of Chirrith are going to show up when a Chirrith speaker speaks in English? Establish your "base" for Language 2 the same way you did for Language 1. (This can actually work for more than two languages also.)
Question 4. How do you work with the two?
The good news is, you may already be done. Once you have the "Language X in English" pattern, then the Vas'her pattern in Chirrith is going to look a lot like the Vas'her pattern in English - because after all, you are writing this all in English. All you need are cues here and there, when they become relevant. When they become relevant will be the moment when the protagonist becomes aware of which language she's speaking in contrast with the other. If the protagonist has lost her memory, this may take a while!
It is important, however, to make the different languages as immediately distinguishable as possible. For this purpose, you might want to consider one more tool: intonation and meter. The distinction between the language concepts and metaphors is going to show, but may not be immediately evident in every line. If you want to push the difference further, consider picking an intonational pattern to associate with one or the other - possibly, in this case, a Vas'her intonational pattern that would mark her speech as different from that of the people around her.
The only thing I don't think you should do - at all! - is try to translate between Chirrith and Vas'her. I've tried translating directly between French and Japanese, and it was confusing and difficult. Especially since both of the languages are products of your own creation, I'd encourage you to move away from thinking of them as languages to be spoken, and start thinking about them as templates to influence your use of English. Relating one variety of English to another is something native speakers do almost on a daily basis. It will probably be much easier.
I encourage responses and questions!