Oh, sorry for that, the question was misunderstood.
I presume that a user-level contract language like Simplicity is going to be translated into low-level byte code like EVM. That low-level code is what gets executed on the blockchain. People can choose other languages with less guarantees, write contracts in those languages, and those contracts will be interoperable with each other. This is how things work right now. In my view the real problem is to introduce a language that will be both safe and expressive at the same time.
I presume that a user-level contract language like Simplicity is going to be translated into low-level byte code like EVM. That low-level code is what gets executed on the blockchain. People can choose other languages with less guarantees, write contracts in those languages, and those contracts will be interoperable with each other. This is how things work right now. In my view the real problem is to introduce a language that will be both safe and expressive at the same time.