I faced the same questions when I was writing The Souled. The MMC speaks only English, the FMC speaks English and Spanish, and the two guides they hire speak Spanish and Quechua. Sounds like a mess, right? But it's manageable. You just have to mention how the interpretation is set up initially, and then maybe mention it here and there as a reminder, depending on how long the dialogue goes on for. Here's a snippet of just a Spanish to English translation:
The travel was mostly easy as the trail was well worn, however there was only room to ride two abreast at times, and more often only in single file, so it was a bit dull as it was not easy to hold a conversation. For myself, needing a translator to speak with either of our guides, I could only converse with Blanc when the path was wide enough to allow it. When we stopped for breaks, which were fairly frequent, I took advantage of the time as best I could to learn more from Kuna.
“Why does Chaski live so far from the nearest village?” I asked her as we stopped for a snack, with Dr. Rojas translating for us.
“The gods prefer this,” was the response.
“Why?”
“I heard Chaski say it is because it’s easier for them to find him.”
“Find him? As in, locate?”
She shook her head as my question was translated, indicating that I misunderstood. “He is closer to the magic that lets him speak with them.”
So, the mushrooms might be nearby. I asked, “What do you know of this magic?”
“Chaski keeps the secret. Only he speaks to the gods. And one day, his apprentice will. His oldest son.”
“How do you know he speaks to the gods?” I asked.
“He can provide answers that he could never know himself. It’s not a trick.”
“What kind of answers?” I continued, as I chewed on a mouthful of sandwich.
Kuna considered this for a moment before offering her reply. “History. That is the best way to learn the truth about Chaski. Ask him of the history of these lands when he speaks to the gods, and you’ll know it to be true. The things he told us about the Inca. It was almost unbelievable.”
So after the introduction of the setup, you can slide into a more typical exchange of words. Here's a snippet of English to Spanish to Quechua and back again, which also turns into a back and forth exchange after the introduction:
I looked at the tea hesitantly, and the big man laughed. He spoke to Kuna, who then spoke to Blanc.
“Chaski says to not be concerned. It is not like the tea from last night. He…made light of your intolerance. He says that this tea is brewed from damiana and coca leaves. It will make you feel better, more alert, and…” she looked down, “make you…more of a man. He says,” she added, ensuring that I knew these were not her own thoughts. I took a sip of the tea and nodded my approval.
“Thank you for your hospitality, Maestro. Your tea is very good.”
This was translated to him, and he simply replied that it was his favorite.
