Back in 2007, there was a distribution Tabuntu that enabled use of Tamil on top of Ubuntu.
How can I similarly use Tamil in a more up-to-date distro such as Ubuntu?
Looks like Tamil support made it into 12.04 or even earlier. Though not everything is completely translated. I would recommend you get in contact with the localization team if you can help them out translating. Translating dialogues isn't very difficult, I have done some translations myself a few years ago for my language.
Last activity for Tabuntu on its sourceforge project page was logged 5 years ago. I consider that project abandoned. However there is an Ubuntu Tamil Team and a group on Launchpad. They seem to have plans for 12.04, I suggest you get in contact with them via IRC or Email. Ask them about the status of Tamil support in Ubuntu and ask yourself what can be done to improve the current status.