wordpress plugin - redirect

I have a simple wordpress with the following URL :

/
/about
/documentation
/article-1
/article-2
...
/article-18

All other URL gives 404, fine.

I would like a simple plugin that create the pages

/fr/
/fr/about
/fr/documentation
/fr/article-1
/fr/article-2
...
/fr/article-18

This new pages would display exactly the same content than the original ones. I don't want a 301 redirect because I want to keep the URL displayed. I just need /fr/documentation to return the same content that /documentation

via Chebli Mohamed