I think it's actually a solid decision from WhatsApp since the majority of their users are from non English speaking countries[0].
[0]: http://www.statista.com/statistics/291540/mobile-internet-us...
One example: WhatsApp is one of the primary messaging services used in Brazil, with something like half the population using it. The vast majority of which don't speak any languages other than Portuguese.
When the courts in Brazil shut down WhatsApp for 48 hours, there was a huge outcry because it is THE primary means of communication for people, especially to talk to family who have gone abroad.