Scripts are always in Scheme. They don't need to be marked as executable.
On Unix-like systems plug-ins are in whatever can be marked "executable" if Gimp has API bindings for the language used: C, but also C++, and most compiled language that can always use the C bindings, and Python for "interpreted" language (there are also Perl historical bindings...). On windows, plugins are binary .EXE (in whatever language can use the API bindings), and there is a special case for Python.
A better way to find where to look for the plugin menu entry is the plugin documentation