I have both packages: firefox and firefox-i18n-pl, versions 59.0-0 and yet FF is still in English. I checked browser’s preferences but the settings page became stupid, like on chrome and there are only language spellchecking settings (set correctly). I also see in add-ons->languages->Language pack pl for Firefox.
Everything seems to be ok and yet, FF is still in English. Something broke or is set incorrectly.
P.S. New firefox may be quicker but it often has issues with some sites. For example, on this forum when I edit a post and mark a word, menu with “bold”, “italics” disappears… On Chromium as always everything is ok.