I think you are confused. Why would anyone want to do that? Yes, language is a big part of the pie, but like I said, that part is solved. If you don't believe it is solved, then please explain what exactly you think is still missing. I don't know exactly what Cortana is intended to do, but assuming it's not entirely different from Siri or Google Now, I can think of a couple dozen things Microsoft yet needs to implement. Speech recognition is not one of them. Like I said, WP has supported speech recognition for many languages for years. That can't be the reason to not role out Cortana globally. If you think it is, tell me exactly why.
I think I understand that better than most here. Again, the investments in speech recognition have been ongoing for over a decade. Also, MS doesn't need to get it working for every language in the world, but how about not using language as an excuse for not rolling Cortana out in those countries where MS already has great support for the local languages... German is just one example.
And finally, how about NEVER using language as an excuse to restrict a service geographically... like I said... there are millions of people outside the U.S. that are just fine using U.S. English on their devices. Even if non-English language recognition was a problem (which it isn't) it still wouldn't be a reason to exclude that group of people from using the service.