Regarding English as the official language, I think the issue is whether government forms should be in one language, or several languages. The classic example is drivers license tests, which I think in New Jersey there are 40 different iterations.

In terms of what the government's policy should be, I think it should be neutral. The federal government should not interfere in State decisions regarding language. States need to decide what to do based on their own needs. The federal government's policy should be English as the primary language, but with some sort of rule of reason for special circumstances, such as using interpreters in courts.

The free market seems to reward language diversity. Vendors who can communicate in more languages can reach more markets. So, in a Libertarian world, I would expect all those private schools to be teaching French, Spanish, Russian, etc.