Show search results only in user's default language

Add a convenient way to show search results only in the user's default language(s).

On Google they let you customize whether you want results in all languages, or only in a number of languages you specify. Something like this would be good for us, too.

Questions to be resolved: what role should the admin have? Should the admin be able to turn the feature on/off? Should the admin be able to customize the feature (like decide everybody sees the results in the one language determined by locale?) Or should we make it simple an just give the user the chance to customize their search experience (providing sane defaults, of course)?

Without having thought of this carefully, I'd be tempted to start a new feature collection that lets the user customize their search experience. This could automatically include language(s) and number of search results to show. We're likely to think of other features down the road.