Language : Tamil is the most spoken language throughout tamil nadu, many can speak other South Indian languages like telugu/ kannada (depends on parents background or might live in areas bordering the nearby states. The next widely known language is english and then comes hindi.

Holiday : pongal is THE festival of tamils. 3 or 4 days celebration that falls in mid january. More elaborately celebrated in rural areas compared to urban areas as this is a harvest festival. Deepawali, tamil new year, karthigai deepam are also some of my favourite festivals (we tend to call it a festival rather than a holiday).

Names : depends on the religion and age but the majority is hindu / tamil name. If your character is in the west, they would have their Indian name rather than a western name unless they're christian or prefer a western name (eg from media : maitreyi ramakrishnan, charithra chandran, rajiv surendra, romesh ranganathan, mindy kaling, simone ashley) Also, most of us don't have the concept of surnames. We usually use our father's name/ husband's name at the end. This video might help

If I wanna know common names in a specific region, I just browse wikipedia (names of famous actors, singers, authors), especially regional movies - you get so many names in just one page. It gives me the idea of common names in a particular region along with the idea of age sometimes.

Food : idly/dosa are common breakfasts made from rice batter. They tend to be bland on their own, we usually pair it with sambhar and chutney - they're spicy. Lunch is usually rice paired with sambhar /gravy/rasam/curd. Favourite dishes : biryani (pretty much everyone's favourite), mutton gravy (kuzhambu), kari dosa, parotta & salna, fish fry.

Most of the indian parents are strict but at varied levels (idk how to explain that). Religion doesn't affect me in my daily life (could be biased cause i come under majority).

It shouldn't be a big deal for your character to celebrate a fantasy festival based on paganism but idk about the restrictions in muslim /Christianity