In most cases, the pilots don't have to speak English. ATC don't speak English per se, they give instructions in terms, and pilots have been trained with these terms before. This is a kind of special case, this happens in ground control, and there are some problems in the ramp, where airplanes do their load/unload/fueling and all that stuffs. The ATC cannot see what is going on on the ground, and ask the pilots in English, and the pilots don't understand.