Korean - The word repertoire of every language alters constantly. It is also rather common for human languages to become radically relexicalized (i.e. adopt new word sets). For example, this is how pidgins (crude languages of first contact) have become creoles (pidgins adopted as a first language by the children of such speakers), then become full languages in many parts of the world. Over the last two thousand years, both Japanese and Korean have absorbed and adapted thousands of Chinese words in their own ways, and according to their own sound rules. The driving motivation for this was probably much the same as the worldwide tendency to borrow English words today. Because of its remoteness however, Japan was never as siniticized as Korea. Much earlier, it seems likely that the invading groups which brought old Altaic-Japanese grammar eventually intermarried with local Austronesian people already settled in the Japanese islands, and absorbed a large proportion of local vocabulary. I am unaware of any similar proto-Austronesian influence in Korea.