Your audience speaks English, Spanish, and Mandarin; your keynoter speaks English. Time to call a translator, right? Wrong. Translation involves the written word. If you want your non-English-speaking guests to understand the key-note, you need an interpreter.
Actually, unless the keynote is less than a half-hour long, you need two interpreters. Because of the intense concentration required for simultaneous interpretation, interpreters work in pairs, alternating every 20 to 30 minutes