Original: Furbys start out speaking entirely Furbish, a language with short words, simple syllables, and various other sounds, but are programmed to speak less and less Furbish and more and more English as they "grow".
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".