Content filtering by language works inconsistently

Steps to reproduce the issue:
- Enable locale module
- Go to admin/content/node
- Apply filtering for a language of your choice.

The content is filtered as expected, but the language option is still available for repeated selection. The type option is removed instead.

This is because in node_filter_form() the type filter is always removed if either type or language filtering is already active.

    }
    if (in_array($type, array('type', 'language'))) {
      // Remove the option if it is already being filtered on.
      unset($filters['type']);   
    }

Easily fixed :

    }
    if (in_array($type, array('type', 'language'))) {
      // Remove the option if it is already being filtered on.
      unset($filters[$type]);   
    }

Patch against HEAD attached.