This is something that gets on my nerves. For the most part, I've found that they allow you to change it in one of the menus. Which can get extremely annoying if you're system is set to Chinese, but generally not so bad.
What really gets on my nerves are things like Firefox that have an option to fix it and won't accept it. My spellcheck has been UK English as often as not for quite a while now no matter how many times I fix it.
Personally, I do think that they ought to give you an easy option to fix it even if most of the time the system language is the language people are likely to want.