Humor across culture and language is extremely difficult. Even in one’s own language it can be difficult, because people don’t find the same things funny. What about essays that aren’t sarcastic, does she still have a problem? Does she have the same problem with reading comprehension in her primary language? I don’t mean just with jokes, I mean in general. My reading comprehensions was about average, while my math and science, were grade levels ahead when I was in school. Reading just was not my thing. I did well in Spanish class too, but reading long essays I never got the full meaning very well. I know if I worked at it I’d probably get better. My SIL’s written English isn’t great, verbally she’s very good, not perfect, but very good, and a very strong understanding. Spanish is her first language. My husband said she writes in Spanish like she does in English. So, she’ll likely never get better at writing in English. I would say stick to subject matter she is interested in for a while to help her improve. This is what we try to do in school now. Get children to read more if they can choose the books that interest them. Also, relate even topics like math to real world experiences for them.