Multi language site not working on sub domain
Ok after several weeks of head scratching. I worked out the solution. So hopefully this will help someone else.

Each locale has it's own folder on the root. So to enable you to view the pages on a subdomain you need to edit the .htaccess file within the locale folder and add a RewriteBase /fr or whatever the folder name of the locale you are targeting is.

This will then resolve the problem.

Hope this helps someone else.