zh_CN or zh-CN for HTML

Hi,

after generating some HTML from XML DocBook for locales consisting of a 
language and a country code, such as zh_CN, I've realized that there's a 
problem with the resulting html document. This is caused by the lang 
attribute of the tags using zh_CN, instead of zh-CN, which is the HTML 
recommendation/standard (see 
http://www.w3.org/TR/REC-html40/struct/dirlang.html#h-8.1.1). Currently, 
I post-process the HTML-files with a string-replace, but I wondered 
whether there's another way of doing this automatically within the XML 
DocBook -> HTML conversion?

Thanks,
Bernd