Yes, Nokia 1 does support the Tamil language.
Yes, Nokia 1 does have the Google Assistant in-built.
Yes, Nokia 1 does have the MicroSD card slot. You can use MicroSD card of up to 64 GB capacity.
Nokia 1 has IP52 certification. It is not exactly waterproof but it does have drip protection.
The Nokia 1 is an inexpensive Android smartphone, and as you can tell from the Nokia 1 review, you very much get what you pay for.