In South India, there is a language called தமிழ். To pronounce it, த is "tha", மி is "mi" and ழ் is "zh". To pronounce "zh", try to imitate a baby saying the letter "r", but add a tinge of the "z".
Anyway, in English, this language is called Tamil. However, the pronunciation dictates that it be called "Thamizlh", "Thamizh", or at least "Thamil". The sound "Th" had been applied in English, so British traders should have been able to convert த to th. Why did they make a mistake and call it "t"? And, as for the letter "ழ்", it was well used in Thamizh, and I do not understand why the British failed to add the letter "zh" or at least convert ழ் into zlh or rlh or zh instead of the least obvious "l". Many fruits, such as மாம்பழம் (mango) have the "ழ்" letter in it. In fact, all fruits end with the word பழம் which means fruit. I do know about the flourishing Indian Ocean trade. When trading fruits and other commodities, the letter ழ் would have been passed on to Southeast Asia, the Islamic World, the Swahili Coast and China. So, when Europeans entered the trade systems during the age of exploration, they would have been exposed to the unique letter. Once they were exposed to it, the started saying it fluently but did not have a letter for it. When transliterating, they would use "zh", although it sounds more like "rlh" in my opinion. So, they were aware of ழ். Why isn't தமிழ் called "Thamizh" or something better than Tamil? Since Tamil is spelt horribly, many Westerners are pronouncing it wrong.
A side question: In other languages, such as Arabic, Chinese, French and Spanish, is தமிழ் transliterated correctly or badly? I would suspect that in Arabic and Chinese, it is transliterated correctly because of exposure through ancient trade.