I know bridges can be built out of metal or wood! I'll just hire an engineer and let them figure it out.
Which is great, because that's exactly what the engineer knows how to do. This should be the same case with the technical co-founder, if not, they're the wrong person to choose as your technical co-founder.
Anybody qualified to be CTO of a company should be able to asses the needs of the software, architect the solution, get up to speed on any technologies they're not yet familiar with, and implement the solution.
That depends on what the idea is. If you're selling software and you're a founder, you'd better at least want to understand your product or you are useless.
Understanding the product doesn't require to know in what language it has been built (unless you're selling a library that can be called from a certain language but I don't think that the kind of product we have in mind here). What you say is akin to a car salesman knowing how the car factory works.
Frameworks and languages have a part in defining the structure of the organization, how development grows, how people's work is measured, and so on. It will ultimately have a significant role in office politics. Once a company scales past 25-100 people, developer politics will be there to stay regardless of language. So you have to pick what kind of trade offs you want to deal with. And a founder should be the informed final word on that crucial choice