<p>I'm wondering which languages support (or don't support) native multithreading, and perhaps get some details about the implementation.  Hopefully we can produce a complete overview of this specific functionality.</p>

Older versions of C and C++ (namely, C89, C99, C++98, and C++03) have no support at all in the core language, although libraries such as POSIX threads are available for pretty much every platform in common user today.
The newest versions of C and C++, C11 and C++11, do have built-in threading support in the language, but it's an optional feature of C11, so implementations such as single-core embedded systems can choose not to support it while supporting the rest of C11 if they desire.