Rewriting a number based URL using .htaccess RewriteRule

Question

How can I rewrite a simple number based URL to a sub folder?

I want http://mysite.com/123 to redirect to http://mysite.com/en/123.

The number could be anything from 1 - 9999. My attempt in htaccess was:

RewriteRule ^([0-9]+)$ /en/$1/ [R]

But that doesn't work.