Problem with langage

[Nizar BOUZID]

Hello,
I am trying to setup an online shop using prestashop, it's multilang store, but for now only english version will work.
I do disable all other langage via the admin page and set the default is english, but when accessing the store, it's always showing the french version (french is disabled!!), i am thinking it's detecting the browser langage.
I do totally delete the french langage to see what will do, and it's ok, the page is in english. BUT the problem is the back office is in english now, and i need it always in french.

Sorry for my bad english, I would like to have english as default langage for store guest (independing from the browser langage) and the back office in french

Thanks a lot.

PS: I do the installation using the french language.