Steps to reproduce the issue:
1. Set the developer mode environment (including portal-developer.properties in your portal-ext.properties)
2. Go to Control Panel -> Documents and Media
3. In the url add /fr/ to change the language to french (you should have something like this: http://localhost:8585/fr/group/control_panel/manage?p_p_id=20&p_p_lifecycle=0&p_p_state=maximized&p_p_mode=view&doAsGroupId=10178)
4. In the Documents and Media paginator you should see that it's being translated to French, but it's not.
However, if you stop the server, and start it in production mode (without any of the portal-developer.properties settings or you just use a bundle) and repeat the steps 2 to 4 you will see that it's translated.