See micronaut-projects/micronaut-core#6960.
Locale as argument is now converted to an accept-language header by a ClientArgumentRequestBinder in
the DefaultHttpClientBinderRegistry
see here.
To make a locale work as path param I tried to register my own ClientArgumentRequestBinder but the one from the DefaultHttpClientBinderRegistry always took precedence.
I solved the original problem by annotation the Locale param with @PathVariable.