Ubiquity wants to force EN language during installation

I'm struggling with Ubiquity for a customized live-cd based on Ubuntu and Xfce. Here's the thing :
I removed all languages except FR (french) from the iso, everything works perfectly if I make an offline installation. Everything is in french, the post-install language tool only shows me the FR languages (french and canadian), without any extra packages to download.

But things are getting wierd when I install with a internet connection, Ubiquity wants to install EN packages, even if I decided to make a french installation. And of course, he wants to download ~150mo of packages.

I looked up in /usr/lib/ubiquity/localechooser/* and usr/share/ubiquity/localechooser-apply, but I can't see where does Ubiquity choose to add EN to my languages.

Does anyone have an idea to prevent Ubiquity to download and install EN languages in addition of selected language ?
I could of course let the EN packages on the iso, but in my opinion, it's a waste of space, since I don't need them.

Thank you !

I found a workaround without having to play with preseeds.

Locate on the iso squashfs: etc/lib/ubiquity/localechooser/localechooser

    # Always support English (unless preseeded otherwise), so that we
    # get English language packs etc.
    if [ "$english" ]; then
        db_set $tpl_supportedlocales "$new"
    else
        db_set $tpl_supportedlocales "${new:+$new, }en_US.UTF-8"
    fi
}

I changed the following lines :

    # Always support English (unless preseeded otherwise), so that we
    # get English language packs etc.
    if [ "$english" ]; then
        db_set $tpl_supportedlocales "$new"
    else
        # MODIFIED, original line is commented below
        # db_set $tpl_supportedlocales "${new:+$new, }en_US.UTF-8"
        # Either or not the language is english, keep only the selected language.
        db_set $tpl_supportedlocales "$new"
    fi
}

It works great.