MS is a global corporation. I don't think the location of their HQ should be allowed to get in the way of good business, and I suspect neither does MS. Anyway, being as I'm in the UK an English version would be fine, no translation needed, but it is still not available which indicates this is not a translation issue.
Ok, first off, you say boot, we say trunk, you say "coop-aye", we say "coop", you say saloon, we say sedan, you say estate, we say station wagon, you say lorry, we say semi-truck, you say caravan, we say camper/RV, you say taking the ****, we say teasing, you say "zed", we say "zee", you say "al-oo-min-ee-um", we say "ah-loo-mi-num", you say pub, we primarily use bar, you say loo, we say bathroom, you say pounds/quid, we say dollars/bucks, you say motorway, we say highway/freeway/interstate, when you say "Asian" it usually means from Southern Asia (Indian, Pakistani, etc), we primarily mean east Asia (Chinese, Japanese, Korean, Vietnamese), etc. We both may speak "English", but it's not that simple. Not only are the accents different, but the regional slang and terminology is as well.
Talderon summed up the rest nicely.
To the business argument, Verizon alone has over 100 Million subscribers, that's roughly the population of the UK and Poland
combined. There's business to be had there (most people are not married to the OS on their phone), and Cortana is a "look at me" feature that can help get people to switch. Business means getting new customers, not just appeasing their existing ones, and the US market has a huge customer base MS would love to tap into.
If I lived overseas I'd probably be bitter too, but their decision absolutely makes sense, from a technological and business standpoint.