"It rains cats and dogs."
अनुवाद:मूसलाधार बारिश होती है।
'Cats and dogs' doesn't literally mean मूसलाधार of course, but it is just an expression in English. These kind of sentences in which you can't make sense by translating each word literally are called 'idioms', or 'मुहावरे' in Hindi. 'It rains cats and dogs' is an English idiom meaning 'It is raining very heavily' or 'It is pouring' or as it is in Hindi, 'मूसलाधार बारिश होती है।'
I think it is important for the course to cover some widely used idioms outside of the special idioms lessons. These are phrases that you will come across and so you will need to know how to use them. As far as I know most course on Duolingo have a sprinkling of idioms throughout the course :)
But if you plan to teach "It's raining cats and dogs" in a section on Present Continuous, it would really make the most sense to teach the idiom then, as "It rains cats and dogs" is hardly ever used, in my experience. Perhaps I haven't had enough conversations about places where intense rain is a regular thing.