Language selector

I have installed my Elgg site with Spanish as main language, English it's not available (don't have EN.php files at all)

So i have Riverdash,CustomIndex,CustomStylez,ProfileManager,Tidy pics and videos too...all in Spanish language...


how can i make to give my users the option to pick English or Spanish language for entire site?...If there's a way, i think i should have EN.php & ES.php for every plugin installed and for the Core too...right?