With SharePoint 2013, and SharePoint Online 2013 (currently still in beta), you can add a language refinement to your search results to filter results based on content language
To do so:
- You must have a search center available (I used the Enterprise Search Center site template)
- Edit the result page (results.aspx)
- Edit the refinement webpart
- Add the languages refiner
- Save and publish your change
Et voilà.
