I am guessing it is because we use the word "Swahili" to refer to both the people and the language, but in Swahili the word "Swahili" means the people and the word "Kiswahili" means the language.
I think the native speakers writing this lesson would instinctively feel that you have to indicate in the translation that you know this sentence is about the language. So they use the term "Swahili language" as the accepted translation of "Kiswahili".
It sounds strange to us because we are happy to say "French lesson" not "French language lesson" and "Swahili lesson" not "Swahili language lesson".
You should probably report it under "My answer should be accepted".
I quite agree - it sounds very peculiar. I think we should at least report it every time they write "Swahili language" for "Kiswahili". (There are several examples.)
"Preparation" might be too vague here though, as even the students do preparation before the class (hopefully). A lesson plan is exclusively done by the teacher.