Question

we said to them , “Hindi is the mother tounge of India". into inderect sentence​

Answer 1

HERE'S YOUR ANSWER MATE:

We told them that Hindi is the mother tongue of India.

Here it is a universal truth, so we will not change the present tense to past tense.

Hope it helps....

Answer 2

We told them that Hindi is the mother tongue of India.(Universal truth)

[ The spelling is "tongue", not "tounge". Hope, it is clear to you.]