title_body: How long do I need to influence language development?
upvoted_answer: Assuming emperor Kai puts a strong pressure on his subjects, having the change permanent would be a matter of one or two generations. Once one generation is brainwashed and use naturally the new form, and that there is no backward trend (emperor Kai's death penalty may be a strong incentive to avoid such trend), consider that adoption should stay.
  
  This however assumes that language is mostly spoken and not written (or you need to change writings as well), that teachers are supportive of Kai and teach the new proper way of speaking to all, etc.