No. You cannot use 2 4G SIM cards at the same time in Nokia 1. One SIM card will work on the 4G network and another will be on the 2G/3G network.
Nokia 1 has IP52 certification. It is not exactly waterproof but it does have drip protection.
Yes, Nokia 1 does have the Google Assistant in-built.
Yes, Nokia 1 does support the Gujarati language. You can also select Gujarati as the system language and your whole menu will reflect in the Gujarati language.
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.