Furbies didn't really "learn" English, they just started speaking more of it over a pre-determined amount of time.

If this one destroys part of your childhood, we’re dearly sorry. If you’re too young to remember, Furbies were wildly popular toys that looked like a bizarre cross between numerous animals. They were notable for their ability to “talk” to one another and gradually “learn” English. These features made them the quintessential toy of the late 90’s. 

They originally launched back in 1998’s holiday season. Within that time, 1.8 million Furbies were purchased. The following year that number rose to 14 million. By the third year of production, over 40 million Furbies were purchased. 

The Furby was so popular because of how intelligent it seemed. If one were to place two Furbies facing each other, they would engage in conversation in “Furbish.” When first purchased, Furbies only spoke Furbish, but the more they were spoken to, the more English they learned. Most considered this to be the most impressive and memorable feature of the Furby. 

Unfortunately, Furbies were just programmed to gradually start using English. They never actually learned it from people speaking to them. After 2000, the Furby slowly died out. There was a brief revival in 2005, but by 2007 Furbies were again hard to come by. If you’re feeling nostalgic or curious, however, then it should please you to know that in Fall 2012, the Furby will be making a comeback. 

(Source)