I think the issue is not language, but Cortana's dependence on Bing services, which are significantly more advanced in US than the rest of the world.
Not just that, but even if you speak english but are native to a different country, unless you speak perfect english, you will have some accent, which will make it difficult for the service to recognize your commands or what you speak.
That would be a frustrating experience, wouldn't it? That's why it's beta first in the US, as they have it improve, get more data for feedback and then gradually roll it out as it gets better.
As i mentioned in a different thread here, this is reminiscent of Kinect Voice Commands when it first launched. Lots of people complained the same way why voice commands were only enabled for US accounts.
If you want it that badly though, having a US account will ensure you'll get it. Most MS services (kinect, xbox live, and likely cortana) depend on the profile you have on your MS account.