Language selector for Apache Solr search form
The Solr Multilingual module should do what you appear to want.


  ... provides a way to offer language-specific searches for different
  languages at once on multilingual websites ...


I'm using the not-yet-released 7x dev version with some success.  A registered-user's primary language selection, or override from a drop-down in the site-wide menu sets a $var which I carry through to the Solr search module, language-specific content, and Drupal language overlays for standard markup, etc.