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.
Control the schools and the culture
Education and culture are critically important to language development. Children of immigrants generally speak the local language, including the local dialect and slang, even if their parents struggle with it. If you can control the schools and the culture, you can force a new speaking style. It's tough to know exactly ...
There is a recently invented language named Toki Pona that claims to be a full language that has about 123 words.
For real spoken languages it seems that Taki Taki only has 340 words.
Language with the fewest words: Taki Taki (also called Sranan), 340 words. Taki ...
You could use Amharic.
1: Region appropriate; supposedly Amharic is ancestral to Arabic and Hebrew. In an alternate timeline maybe it stayed.
2: Alphabet is unfamiliar looking to Western readers and I bet Arabic / Hebrew readers also.
3: It is in Google Translate which helps as regards making stuff up. Your text:
እኔ ሙሉ በሙሉ አዲስ ቋንቋ ከመፍጠር ...