manifest: specify full install-langs list
    
    This is the same change as what was done for the master branch in
    https://pagure.io/fedora-atomic/c/5a5eddd?branch=master but for F25. We
    can do this now that an rpm-ostree which properly understands
    install-langs (https://github.com/projectatomic/rpm-ostree/pull/525) is
    in the stable repos.
    
    This is slightly different than what we had before, because we no longer
    prune non-UTF8 locales. Since they're minuscule in comparison, I'd say
    it's a much nicer benefit to not have to maintain any logic in
    treecompose-post.sh.