When you are on a manual webpage, and perform a search 
using the form in the header of the page, you will be 
directed to the page or function in the manual of the 
default language of the current site. This default language 
of a site may not be the same as the language the user had 
chosen earlier.

Example:
At http://www.php.net/manual/nl/, do a search on 
'phpinfo' and you will end up at
http://www.php.net/manual/en/function.phpinfo.php
Effectively switching from Dutch (nl) to English (en)

Or, at http://nl.php.net/manual/en/, do a search on 
'phpinfo' and you will end up at
http://nl.php.net/manual/nl/function.phpinfo.php
Effectively switching from English to Dutch.

Possible fix: just add a 'input type=hidden name=lang 'tag 
with the current language in the CommonHeader.