My English teacher told me that proper nouns like "Jonathan" can be pronounced in anyway you like. Is that true?
I think it's always best to pronounce proper names in the same way as the "owner" of the name. So if you have a friend named Jonathan he is free to pronounce that how he likes but you should pronounce it the same way to avoid causing offence. (Although it's hard to think of more than one way of pronouncing Jonathan!)
The same is true of names of towns and cities where it is best to pronounce the name in the same way as the locals.