Learning another language is difficult and being completely fluent in two or more is a highly useful skill. While there are professional and educational benefits to being bilingual, you can also be something of an amateur spy.
Is there any better revenge than getting back at people who were talking about you behind your back? If people think you can't understand them, they can be pretty reckless and loud with their conversation. And sometimes, it can get nasty...
My mother tongue is Arabic, I look very Middle Eastern, but I speak very decent Russian and this was in Dubai. Was in an elevator in a fancy hotel and a Russian speaking couple walk in, the lady was visibly upset. The guy was apologizing and saying something along the lines of 'c'mon was it that bad?' And the lady says 'i told you before not to cum in my mouth'. I couldn't help it and laughed, told them I could understand and apologized. The lady looked at me in horror and disbelief as the guy laughed his ass off.
That was an awkward wait till I hopped off the elevator.