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Why do biblical names vary in different languages?
We call him Jesus in English. But in the Tamil language, the word Jesus is pronounced as "Eeasu". I wonder why such thing is happening. For example, if a person's name is John in English, he will be called John in Tamil and hopefully in other languages too.
But why does this rule differ in case of Jesus Christ?
Your premise that other names are said the same across languages is simply not correct. Lots of writing systems and spoken sounds don't have one-to-one equivalents in other languages, and over time it is quite common for names to morph to things that are easier to write or say. Sometimes the name will be spelled the same but pronounced locally, then somebody will come along and start writing it how it sounds and so on. Sometimes names have meanings and it is common to translate the meaning into a new name. Then that name gets transliterated and the process continues.
The main point is that Christianity's use is no different than common language practice. If anything there are some very common threads that connect all the dots—more so than you might find for just any name.
A simple example is my own name. Caleb on English is neither pronounceable nor spellable in Turkish. They can't actually say it well even repeat-after-me style because they never end words with "b" and the other sounds just don't go together well. If they see it written, they say it differently entirely. Usually I go by a rough sound equivalent and don't worry about how they spell it. If I'm doing documentation of any kind, I go by the spelling and just introduce myself with their pronunciation of that to save trouble, and I am accustomed to answering to that name as well. The other alternative would be to use the name as it had come to be translated in scripture, which through hand-me-down tradition and various language transitions is rendered "kalev". Incidentally this is actually closer to the Hebrew version of the name than my given English name.
For reference, Jesus is rendered as "İsa" in Turkish.
I agree with Caleb, just to elaborate a little:
Languages don't always have the same sounds. When people who grew up with one language try to learn another, they often have problems with sounds that are used in the new language but not in their native language. For example, Chinese people trying to learn English often say "r" rather than "l", hence all the jokes about "Engrish". Native English speakers have trouble with the guttural "kh" sound found in other languages: we constantly struggle with whether to render it in English as "k" or "q" or "kh", etc.
Amusingly enough, there's an example of this in the Bible: Judges 12:6. In a war between two groups of people, the people on one side were able to tell that someone was a member of the other by asking them to say the word "shibboleth". If they couldn't pronounce the "sh" sound and said "sibboleth", then they knew they were one of the enemy. One can learn new sounds with sufficient time and effort, of course, but probably not when someone is standing with a sword to your throat demanding you say it NOW. This is probably not the ideal learning environment.
As Itpastorn notes, "Jesus" is not really very close to how Jesus said his own name. Nor is "Jehovah" all that close to the name of the father. It's probably just as well that we don't know how to pronounce God's name correctly: People would just use it as a swear word.
