Communication is fundamental to the human experience, so this issue of language can make for an intriguing complexity in your story. When dealing with a foreign language in fiction you have the possibility for two circumstances: a character who is unfamiliar with the language and a character who is familiar. In both cases, it’s often wise to assume the reader has little or no familiarity. Your story has a little bit of both, so let’s take a look at the different techniques.
When a character is unfamiliar with the language, introduce it in the way he would experience it: What does this language sound like? Is it sharp? Musical? Flat? This can help build atmosphere as well. A beautiful sounding language, for example, might take on a haunting quality to a listener who is frightened and unable to understand what is happening. Much of communication is found in body language and facial expression, as well. What words or phrases can he identify as a result of these non-verbal cues? Showing the character make these kinds of connections can also help the reader understand how he begins to learn the language.
When a character is familiar with the language, the author is faced with the additional complexity of how much of that language to include in the story. Too much can make the reader gloss over large sections and lose meaning or clarity. Too little and the reader doesn’t get a real sense of the quality of language. One effective solution is to give the dialogue in English, but describe which language is being used. Chimamanda Ngozi Adiche does this in her novel Purple Hibiscus, where her Nigerian characters speak in both Igbo and English. She uses the choice of language to further characterize. Kambili’s overbearing father, for example, uses English at home to help teach his children, but often reverts to Igbo when around his sister. Kambili’s grandfather does not speak English and the choices the characters make about which language to use while around him can be quite revealing.
You can capture the flavor of the language by peppering in some words that can be gleaned through context. For example, Kambili’s aunt says, “Kambili, I want you to help me do the orah leaves, so I can start the soup when I come back.” The reader may not know much about orah leaves, but the context gives the necessary details. Other times, words are translated: “The women clapped and hooted when Jaja and I said, ‘Nno nu.’ Welcome.” You might keep this limited to short and simple words or phrases, so as not to weigh down the prose.
With the right guidance you can help the reader effortlessly experience the unique richness of the unfamiliar language.