Hi sgunhouse, thanks for reply.
This is definitely not a language, I believe. My OS X language is set English(US), and I live in China now.
I tried Opera and Opera Beta, both work well, but when it comes to Opera Developer, everything went wrong.
I have add English in my Opera Developer, but no work. On Windows, it's easy to change Opera UI language in this section, but on OS X, there is no option for that, just
Change the default spell check dictionary or add new dictionaries
Dictionaries... (a button)
Specify your preferred languages for webpages
Preferred languages... (another button)
So I can't change UI language. I reinstall Opera Developer, and still have this problem. As Opera and Opera Beta do not have such issue, maybe a bug?