phranque - 10:57 am on Mar 15, 2013 (gmt 0)
in the RewriteRule pattern use a start anchor before the "en" and an end anchor after the "html".
use parentheses to capture everything from the "/" to the "html" (inclusive) so you can use that as a back-reference in the substitution string.
be sure to include the full canonical protocol and hostname in the substitution string and use the [R=301,L] flags for your redirect.
put this rule before your general hostname canonicalization redirect.
(they should be ordered most specific to most general.)