What you might do is something like this:
Code:
<form action='http://validate/phpdig/search.php' method='get'>
<input type='hidden'name='template_demo'
value='custom.html.en'/>
<select name='phpdig_language'/>
<option selected value = ''></option>
<option value="en">English</option>
<option value="de">German</option>
<input type='hidden' name='site' value=''/>
<input type='hidden' name='path' value=''/>
<input type='hidden' name='result_page' value='search.php'/>
Then of course, your config.php file would need to select a default language, but you said you're already doing that.
Hope this helps.