Or in the Japanese example, even supposing it were true, I would expect some other formalized way of showing gratitude; hence "thanks" the concept would live on even if there was no word used in similar contexts to English "thanks."

Very true. "Thank you" doesn't really have a meaning the same way other words do, since it's more of an interjection. When finding the translation for "thank you" in other languages, you just look at what the recipient of a favor says to express appreciation in that language, and call that their "thank you".

Otherwise, you could argue that "Spanish doesn't have a word for thank you -- but hey, on an unrelated note, native Spanish speakers have this odd custom of saying "gratitude" (gracias) whenever they want to thank someone..."