In Joomla:

$mainframe->getCfg('lang');

will return "british" (i believe this can be a default value), anybody will easily notice me where defined it?

Can One change it out?

It's that which you configure within the Language Manager.

In Joomla! 1.5, visit Extensions > Language Manager. This can list all the languages presently placed on the body, such as the default (which you'll set).

However, customers may also choose the word what they would like to use. The code $mainframe->getCfg('lang') will return the word what selected through the user rather than the default.

Also, you need to stay away from the $mainframe object and rather retrieve the present language using JFactory:

$lang =& JFactory::getLanguage();
echo $lang->getName();

jlleblanc,interesting help. i believe i've found the issue: its language files' problem,just alter the backwardLang in metadata,Listed here are the codes:

<metadata>
<name>Simplified Chinese</name>
<tag>zh-CN</tag>
<rtl>0</rtl>
<locale>zh_CN.utf8, zh, zho, zhi, zh_CN, zho_CN, zhi_CN, simplified_chinese</locale>
<winCodePage>cp936</winCodePage>
<backwardLang>simplified_chinese</backwardLang>
</metadata>