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

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  • Add the languages refiner

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  • Save and publish your change

Et voilà.

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