Yeah just as I thought, it would be super easy for BC to implement this server side with a where statement in the SQL but BC just doesn't do multiple languages very well, their implementation was poorly planned and obviously from a person that didn't understand the complexities of multiple languages.
The only way to do this client side would be to filter results in AJAX but it would be super super messy and there is no guarantee BC would give you the results you wanted (in the language you wanted) first time, so you would have to loop the search results in AJAX page after page until you got enough results to fill a page (of the language you wanted), then repeat this exceptionally for further pages. Messy messy, do not do this!
My recommendation, use google search for websites, you wont get the fine details or customization of results that BC gives you, plus you might get ads, but it will work PER unique domain name and give the results back in that language.