This seems like a "see three, pick two" problem. Compulsory education in the standard language will eventually solve most of these concerns. Ready understandability by elders seems quite hard to achieve, and might only possible if the new standard is quite similar to an already widespread register. And that's not even addressing the issue that daily life of some ethnic subgroups might happen in a completely different language, who might also be actively resistant of assimilating into the national culture.