Base on my readings of this thread it seems as if the OP is suggesting the Redmond to change their primary language of operations to a language in EU example would be Italian or Wales. Code in these languages, push out to these countries first and then work their way back home.
I live in Switzerland. I once developed software. Comments, source code and documentation were never in any other language but English. My point is that MS doesn't have to change anything about their language of operations to develop software for other countries. That is pretty much irrelevant. Furthermore, during my time in software development, the applications we worked on were released in at least three languages simultaneously. If we can do it, MS can do it too.
Finally, a huge part of the European population speaks more than one language. Most people I know simply don't care what language Cortana comes to us in, as long as it gets here, fast. Many would actually prefer English, despite that not being our native language. I also know many people in the Netherlands and Germany who think similarly.
Cortana aside (based on language differences) you can't really argue the facts. The US is slow to adopt and MS should invest where the real money and interest is.
Cortana initially being U.S. only has little to nothing to do with language. I've used MS speech recognition technology for over a decade in many languages other than English. The technology has existed almost forever already. If language was the reason for the hold up, you'd also have to explain why Canada, Australia, and the many other English speaking countries are not part of the rollout.
The reason is Bing, and the fact that in order for Bing to answer questions, it must be hooked up to national data sources. You can't track flights from non-U.S. airlines, if U.S. airlines are the only companies hooked up to Bing. You also can't reserve restaurant seats in Switzerland, make concert bookings in Germany, or answer questions about French national monuments, when Bing knows nothing about the world outside the U.S. That is what's holding up Cortana's international debut.
Despite all this, I don't think changing emphasis is a good idea. WP's biggest problem is the app gap, and most of the companies that provide these mobile services and apps are U.S. based. I think MS must have more success in the U.S. in order to have success world wide.
IMHO there is nothing wrong with servicing the U.S. first. What is problematic is the amount of time that passes before those services are rolled out internationally. Often times some services don't get rolled out at all (podcasts, audio search, xbox music subscriptions, etc). By the time Cortana reaches mainland Europe, we will be well into 2015, maybe even close to 2016. Cortana will be old news. At least for Europe, Cortana will be irrelevant in terms of marketing affect. That is a shame, considering all the time MS invested to build it.
A lot of people I work with view MS as a backwater U.S. company that just coincidentally does some business oversees, whereas Google and Apple are truly international companies. Apple rolls out most of their services in Switzerland just as fast as they do in the U.S. Apple have even integrated features specific to Switzerland into iOS. Is it any wonder that Apple commands more than 50% of the smartphone market share in Switzerland?
TL;DR:
IMHO the U.S. must remain the first country MS targets, but they must also be able to role out their services world wide within a matter of months. Not years. In a nutshell, more emphasis on rolling out new features everywhere faster, not a reversal of emphasis.