Missing Language under Styles Library

Hello,

I installed a SharePoint 2013 Server, I created Sites Collection.

I installed the French Language pack (rerun wizard etc...) two weeks after.

Now when I want to create a new page layout (with the Computer in French language), CSS is not applied. After some tests, I found that the fr-fr folder is missing under the Styles Library.

If I create a new Site Collection, this folder is correctly added.

What can I do to automatically add this folder under all previous existing Sites Collection? I'm a bit confused with the Language Pack installation.

Regards,