Why is MS damned either way? I don't think they are. People who can't speak English well enough to use Cortana typically have little difficulty accepting that getting other languages to work is difficult and takes time. It just shouldn't take years.
However, if I set my OS and speech setting to the same language, and if Cortana supports it, then Cortana must run. There is no excuse for her not to, because the user just told the OS that he/she is proficient in that language. It shouldn't matter where that person lives, which units of measurement that person uses, or how they like their numbers formatted on on screen!
There are many people who want Cortana to work like that. For example, I live in Switzerland, were 25% of the population were born elsewhere! That explains why English is very common here, even if it is not one of our official languages (of which we have four). I work with dozens of people who were born and raised in the U.S. or the U.K. Is MS assuming they all must have forgotten how to speak their native language? I grew up here, but I also prefer my technical gear to be setup in English (shorter words, easier to use the correct terminology for internet searches, etc). Cortana understands me really well too. That I can't use Cortana without having to set my region to the U.S. seems like a very artificial restriction that serves no purpose.