French Translations not updated for the messages and button in Extension Manager - Adding extensions

Steps to reproduce:

1. Log in as Admin
2. Go to Administer Wiki
3. In Localization set Multilingual to No, Supported Languages to French and # Default Language also to French
4. Click Save
5. Click on "Ajouter des extensions" from Gestionnaire D'Extensions
6. Observe the message and the button, in the top of the page
7. Search for an extension that is not in the recommended list e.g. Lightbox
8. Observe the message

Expected results: The information/warning messages are shown in French, as selected, and so are the buttons.

Actual results: The information/warning messages and button remain in English.