No one will want to write GUI code that directly parses and generates XML, so every language will end up putting wrappers around that, and you are back to where

No one will want to write GUI code that directly parses and generates XML, so every language will end up putting wrappers around that, and you are back to where you started with an expensive translation layer in-between. Having a machine format that describes the interface and facilitates auto-generating the bindings, like GObject introspection is a better option and exists today. Even then the auto-generator will need to be custom for each language, to handle integration with the language's standard library data structures, memory management and the like.