The source MIT Technology Review article seems to answer your questions
Earlier this year, OLPC workers dropped off closed boxes containing the tablets, taped shut, with no instruction. “I thought the kids would play with the boxes. Within four minutes, one kid not only opened the box, found the on-off switch … powered it up. Within five days, they were using 47 apps per child, per day. Within two weeks, they were singing ABC songs in the village, and within five months, they had hacked Android,” Negroponte said. “Some idiot in our organization or in the Media Lab had disabled the camera, and they figured out the camera, and had hacked Android.”
Elaborating later on Negroponte’s hacking comment, Ed McNierney, OLPC’s chief technology officer, said that the kids had gotten around OLPC’s effort to freeze desktop settings. “The kids had completely customized the desktop—so every kids’ tablet looked different. We had installed software to prevent them from doing that,” McNierney said. “And the fact they worked around it was clearly the kind of creativity, the kind of inquiry, the kind of discovery that we think is essential to learning.”
So, learning English consisted, at least, of learning to sing the Alphabet song.
The hacking consisted of working around an OEM attempt to limit the settings. Whether you consider discovering and using a bug to unlock existing functionality to be "hacking" depends entirely on your definition of "hacking". It is quite different from, say, developing a technique to "jail-break" an iPhone or rebuilding a kernel.