So IATL Manchu is a moribund language spoken by 60-70 elderly people. This is bad. Challenge: Manchu a vibrant language spoken in the (non-extreme) northern regions of China. Bonus points, make it a language a Mandarin-speaking native would want to learn. Extra bonus points, awesome Manchu diaspora.
Time maximum is the Qing ascendancy. Failing that, whenever.