Try replacing all of that with just this:
RewriteRule ^eng/(.*) en/$1 [R,L,NE,DPI]
If it works, you can change the
To test this, you must use a new browser. This is because browsers cache
301 redirects, so your current browser will keep redirecting to the old
(wrong page) without even talking to your server and hitting the new
.htaccess—unless you clear the browser cache.