I have my site broken down into several folders by language:
/ /en/ index.php about-us.php faq.php ... /fr/ index.php about-us.php
Put this rule before your rule:
RewriteRule !^(fr|en)/ /en%{REQUEST_URI} [L,R=301]
Or more general:
RewriteRule !^[a-z]{2}/ /en%{REQUEST_URI} [L,R=301]