Is C++ the only language that I can use for Premiere Pro CC plugin development?
Someone told me that Flex is also possible, is that true?
And how about C#?
Ahile the API interface is actually c (it's is "extern "c" ..."ed in the headers) there is no reason why you can't then jump to another language.
For example, my dialog boxes on the Mac version are ObjC and C++ on the Windows version.
There is no reason why you couldn't call C# functions (you just have to always export the api as "c") so that the calling convention is correct.