It's weird for most people. We're used to cross-language wire protocols. OpenTracing is different.
An analogy is SLF4J for Java logging. All libraries, etc use the same interface and the final user determines the backend: java.util, Logback. This makes sense if you have many authors of libraries with a cross-cutting concern.
This really makes OpenTracing half a dozen different standards, one per language, with common semantics.