I had the same on a Tube, 2 portuguese lads were discussing the entire carriage, and I was sitting opposite them, reading my paper, just waiting for my turn to come, which of course it did.
I waited until they'd finished, just as the train pulled into my station before reminding them that many people are more than what they seem to be...
Their faces were priceless...
I've messed with brits tho too, trying to guess the language. we weren't saying anything bad, just chatting about everyday stuff... My friends and I spoke portuguese with other accents, just to mess with them, and I only revealed that I was as british as they were upon leaving the train... that was a laugh too...