Best language other than English to communicate with Ugandan student?

My school is sending letters to students in Uganda this week, and I'm aware that English is the official language, as well as Swahili. I wanted to incorporate phrases in another language into my letter, but I obviously want to ure that the student who recieves my letter speaks the language. What language should I use? I don't know what region of Uganda they are from, so it needs to be widely spoken, like English, Swahili, etc. What language is best?