Tamil (தமிழ்) has its own script. The 2011 Census counted 6,90,26,881 people with Tamil as their mother tongue, 5.70% of India and the fifth largest group. Tamil Nadu had 6,37,53,997. Karnataka had 21,10,128, Puducherry 11,00,976, Andhra Pradesh (as in 2011) 7,13,848, Maharashtra 5,09,887 and Kerala 5,02,516. The Census counts people in India only.
The 17.5% is for read sentences. A Whisper model of unstated size scored 78.4% in the IndicVoices test, where most speech is unscripted. Whisper’s paper finds that a language’s error rate tracks its training hours closely, and Tamil’s 136 hours put it well ahead of Hindi’s 12. But its test sets are read sentences, so the 17.5% says little about unscripted meeting speech.
If your staff mix English into Tamil sentences, test the mix. None of the sources reports mixed-language Tamil results.
For English meetings, the Svarah paper measured Whisper large at 5.3% word error rate on speakers whose mother tongue is Tamil, the same as native Hindi speakers (5.3%).