Thanks to a girl from the assistance service I got the rewrite rules to work as I wanted. My problem was that under the covers they had MultiViews enabled, hosting service side, so I addedd the following line to my .htaccess to disable it: Options -MultiViews.

I did some testing and found the evidence of what is written in the documentation, I tried requesting /some/dir/foo and then got redirected to /some/dir/foo.html; as explained here I requested /index and got index.html.en in response, this seems an useful way of providing i18n content with the same url, based on browser locale.