Ironically, I picked C++ to implement, having no idea that it was unimplementable :-)
(It took until the aughts for EDG to be the first to do a complete implementation of C++.)
It is difficult to think of Ada as a 'large language' compared to everything else. I'd love to see Ada and Modula-3 have a resurgence.
There wasn't anything particularly special about it when I last used it in the 1990s. It wasn't a bad language, but there wasn't anything particularly compelling about it or its implementations, either. There was nothing that really made it more useful than C++, or even Delphi and Java.
Modula-3 predated Java by about 10 years and I'd say the type system [1] still surpasses it. Modula-3 was nearly everything that Java ended up being, a safe performant systems language.
[0] http://en.wikipedia.org/wiki/Modula-3
[1] http://www.cs.cmu.edu/~rwh/courses/modules/papers/cardelli-e...