"Mwalimu anafundisha somo la kiswahili"
Translation:A teacher is teaching the Swahili lesson
This is not correct usage of the word "subject" in an English sentence. The question may certainly be asked: "What subject do you teach?" BUT the answer is "I teach Swahili" or "I teach the Swahili course" (assuming there is only one). Native speakers of English do not say "I teach the Swahili subject."
Regarding the Swahili sentence, is this use of "somo la .." obligatory? Is it a mistake to say, "Ninafundisha Kiswahili" or "Ninafundisha hisibati", leaving out the "somo la"?
no, you can leave 'somo la' out and it wouldn't change the meaning. I think here the intended meaning is to distinguish it as a formal course; in East Africa we call courses, subjects. But I agree, it's more common to leave it out