You're not thinking about the issue correctly, and although it's already been said, I want to reiterate:
Language and country are not related in the way most Americans think. The country I live in has four official languages, none of which are English. Nevertheless, most of my fellow citizens have no problem understanding or speaking English. Consider also that MANY U.S. citizens also live and work here... they speak English like any other American and use the same services... why exclude them from using Cortana? In fact, most people I work with would prefer to use such services in English. The days when people were fluent in only a single language, and national boarders defined where the use of one language stopped an another started are long gone. Only Americans still have difficulty grasping that fact... international companies do learn however... Apple has learned, Google has learned... for some reason, MS is still clueless.
The point? Being restricted to U.S. English should have zero influence on whether the service is launched globally or not. The only thing that should matter is the language you have set your device to. Instead, MS typically disables services for everyone who's Microsoft Account has a country code other than "United States". Microsoft has a long history of disabling services outside the U.S., even if language is completely irrelevant (like Xbox Music or Audio Search). I think it's obvious that language support has nothing to do with this, particularly since MS already has the tech in place to support a whole host of languages.
Like others said, the issue is more likely that Bing doesn't stand on equal footing outside the U.S.