Imagine that you, a non-English-speaking person, are writing a letter to your family in China. They, too, do not speak English. What language are you going to use to communicate with them? Spanish? Why not? It’s the second-most spoken language in the world, behind Mandarin.
We don’t know the slightest bit of Mandarin, but we can tell you that this letter is more historically accurate than what the teacher was expecting. Also, since this was set in 1870, you can bet that there wasn’t fair treatment for people from China. We’re going to assume that there are a few select cusses in this letter.