You can enable Tamil keyboard support by doing either:
open regional and language
in input sources add Tamil
An icon should appear to right, giving you the ability to switch between languages.
You can of course also reach settings from the drop down menu top right.
We really need a wiki page to cover enabling support for Indian languages, will try and get some effort devoted towards that.
Offering keyboards for sale via the our upcoming online store would also make a lot of sense. Do you know if Tamil keyboards are readily available - or do people use some sort of keyboard overlay?
Keyboard stickers? now that's a DIY approach that would be very much Hamara