"We will not learn Swahili language"
I'm not sure if it's idiomatic to say "language of Swahili" like this... it probably is but it seems a bit redundant because the ki- makes it pretty clear.
In any case, lugha is an N class noun (9/10) so it would be lugha ya Kiswahili.
And also, the question "Why is ... rejected?" very often has the answer: "Because it's a beta course with a lot of work to be done. You can of course ask questions in the forum to learn more, but you can report things like this to help the course improve, eventually, when the creators get time to work on this.