How can the well-known Tintin become タンタン rather than ティンティン in Japanese? Is it because ティンティン sounds like ちんちん?
The タンタン rendering has a lot to do with the source French pronunciation, [[tɛ̃tɛ̃]]. The [[ɛ]] vowel in the International Phonetic Alphabet is the open-mid front unrounded vowel, halfway between [[a]] and [[e]]. The tilde ~ on top indicates nasalization, like a half-pronounced [[n]] after a vowel. To a Japanese ear (and, heck, even to my American English ear), [[tɛ̃tɛ̃]] sounds not too far from タンタン.
Avoiding the pronunciation [ちんちん]{chin chin} (slang for "penis") is a nice side bonus.