"They learn Swahili"
Formal would " wao wanajifunza Kiswahili." The program asks for a direct translation and provides an informal response not a direct translation
Omitting "wao" is not "informal". It's natural. You'd only include it to emphasise: "THEY learn Swahili." I agree that both should be accepted, but the pronouns are used in Swahili for emphasis - it has nothing to do with formality.