How to weak certain CJK fonts under zh_CN locale

[Zhangsan]

I installed YaHei fonts in Fedora 15, which are the only CJK fonts.
When LANG=en_US.utf8, fc-match monospace returns "DejaVu Sans Mono",
which are the default mono fonts. When LANG=zh_CN.utf8, fc-match monospace 
returns "Microsoft YaHei"。I don't want this result, because ASCII 
characters are not fixed width in YaHei and my desktop seems very ugly
at many places. Although I am able to adjust fonts to these places 
separately, I think add a file to /etc/fonts/conf.d is more effective.
I read fontconfig man pages, which says that
font family names are split into two bindings; strong and weak. Strong
family names are given greater precedence in the match than lang
elements while weak family names are given lower precedence than lang
elements.
But it doesn't say how to set such weak bindings. I need some help, thanks.
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