So, from what I understand, living things maintain (or at least prior to the industrial revolution did maintain) a predictable ratio of C-14 to C-12. I'm not super familiar with the mechanics of this, I imagine it's a case of the amount of C-14 lost matching the rate it was replaced via respiration.
Once the organism dies, it stops controlling that ratio and we can measure the decay using a sample of the material.
Japanese to English is pretty tricky, in part because it's more standard in Japanese to fully omit words instead of using a pronoun when something is known from context. A response to "What are you doing with that book?" in English might be "I'm going to return it," but in Japanese you'd just say "Return" (返す。). So a machine translator would probably have to be very good at context to guess right a lot of the time.