Original: A newly purchased Furby starts out speaking entirely Furbish, the unique language that all Furbies use, but are programmed to speak less Furbish as they gradually start using English.
Simplified: A newly bought Furby starts out speaking entirely Furbish, the unique language with short words, simple syllables, and various other sounds, that all Furbies use, but are programmed to speak less Furbish as they learn more English as they "grow". Throughout a Furby's lifetime, it gradually learns English words and phrases, which it begins to speak in place of Furbish.