The protocol itself is really only language-specific to a similar extent that JSON is language-specific. Which you can totally argue it is, but also people have figured out how to use it in lots of languages other than JavaScript.

I think Cap'n Web could work pretty well in Python and other dynamically-typed languages. Statically-typed would be a bit trickier (again, in the same sense that they are harder to use JSON in), but the answer there might just be to bridge to Cap'n Proto...

Retrofitting to some but not all languages is not nearly the same as an intentionally language agnostic protocol. To the extent that calling this "web" is at best misleading.