It will be literally impossible to have one language that all chains can understand, like there is no single programming language that every platform can understand. But some of them can have common language and thus inter operable. Its definitely desirable to have interoperability across all chains, but I think, its just not possible.
I think it will be a gradual change and it might be a 2 or 3 layer approach like the lightening network so instead of trying to force or match every chain to speak to one another it speaks across chains to find the quickest route to get the job done!
I think there are a range of ways to provide a solution
Ya, I think, if each chain provides some sort of API based access for Interoperability, then that would work. So its not necessary that they need to have same language, but they only provide the interoperability.