This is not an uncommon problem, and is usually dealt with by judges in different ways. For example, the Dineh (Navajo) language is so complex, that a translator at a trial may say “untranslatable”, repeatedly, for a minute or more before the judge asks for a re-phrase, until they discover some means of expressing the testimony in English.
In this case, the judge could order an expert in similar African languages to try and set up a dialogue with the defendant, to be able to convey the major concepts of the trial to him. Though circumstantial evidence will be essential, as well as the testimony of other witnesses, it should be no harder to try the defendant that if he were being tried in absentia.
The rapist speaks and understands English. He went to school in Maryland and even graduated from Highschool and went to a Community college