Excellent plugin. Nice and simple.

For my purposes I've added a rel tag to include the $language so I can use it as content in the ::before pseudo and style it on the front end.

Line 245:
sprintf( ' rel=%s class="%s" ', esc_attr( $language ), esc_attr( $added_classes ) ),

Worth doing? It's a slight misuse of the rel tag so if you can think of a better solution...