"Mari kita belajar bahasa Indonesia" should be accepted.
How about "mari belajar bahasa Indonesia"? Any reason that should not be accepted?
AFAIK, no reason at all, "mari belajar …" should be fine.
Doesn't Indonesian require a preposition after an intransitive verb like /belajar/ in order to be correct grammar? How is an intransitive verb taking an object?
Otherwise, the sentence could be /Ayo kita mempelajari bahasa Indonesia!/ and it would be perfect.
Why is "Ayo kita belajar bahasa Indonesia," incorrect?