I don't think you'd have to ban unsafe languages but you could pass a law that slowly decreased the amount of money the government could spend on 'unsafe' software (either through direct licensing or renting through clouds). The government is such a huge client to these companies that it'd immediately create a large financial incentive to migrate.
I think this is probably one of the better, more practical ideas that I've seen. If the government has to consider whether the underlying technology has adequately addressed memory safety issues (doesn't have to be at a language level, but that's obviously the easiest way), that puts pressure on them to fund projects that use memory safe approaches.
That's billions of dollars that will get slowly steered in the right direction.
