I have to route

http://domain.fr/subpage

to

http://domain.com/fr/subpage

using htaccess. Any ideas how?

All of the small config aspects of .htaceess are driving me batty about this cold Monday morning.

Do this:

RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^(www\.)?domain\.fr$ [NC]
RewriteCond %{REQUEST_URI} ^/subpage[/]?$
RewriteRule (.*) http://domain.com/fr/subpage [R=301,L]

Individuals rules will see if the domain is domain.fr and when the URI is /subpage or /subpage/ and can then redirect with statuscode 301 (permantly moved) towards the actual address you want.