Programming

it may be underestimating how tightly coupled syntax and language is.

From a CS perspective, syntax is a major part of a programming language. A syntax (or formal language) is defined essentially as an alphabet and the sequences of words constructed from that alphabet that are allowed in the language. Programming languages combine the syntax, which can be defined formally through BNF (for example), with the semantics they enforce. Semantics include things like "types", "modules", or even "lifetimes" potentially.

Where you can try to decouple it is between the syntax and the semantics. In fact, this is not without precedent. For example, the JVM and the CLR both allow arbitrary syntaxes to be compiled and executed on them, and both have many languages that can be seamlessly integrated together that all compile to the same intermediate representation. Here's a project compiling Rust to the CLR if you want an extreme example, though more practical examples for the CLR would be C#, F#, and Visual Basic (which can all use each other's defined types, methods, etc).

So basically, what you might be looking for is a common "intermediate language" for languages. We actually have one already though, and that's the C ABI. I think a better ABI could exist though, ideally one which allows more information to be shared across boundaries. Still, that's where I'd start looking more into this, maybe with inspiration from the JVM or CLR.