Before anyone from t'other side of t'pond reads this, please, please note that I have many American friends! I do not doubt their intelligence, nor do I take the piss. It was just one bloke who was thick as a brick I'm having a pop at, alright?

A fair few moons ago, I worked as a waitress in a hotel in Switzerland.
It was a very nice hotel set atop a small mountain, and afforded splendid views of the valley below, some other mountains, and on occasion cows.
Being such a very nice hotel it was generally rather busy, but Tuesdays were normally nice and quiet with only a small amount of annoying pleasant customers demanding coming to eat and drink there.
I liked Tuesdays.
The hotel was rather old and to get to the bar you had to climb lots of stairs. Not too bad if you were a customer and could sit down and relax after, but hideous for a poor waitress having to run up and down them carrying large trays of beverages.
One fine Tuesday, I was busy rearranging the dust and plotting my plans for Total World Domination, when I heard a car pull up outside. A quick glance out the window revealed three customers, so I cleared away anything incriminating and tried to look pleasant and welcoming.
They came up to the bar via the back stairs which also meant they had to walk through the 'posh' restauranty bit to get to me. They began to loiter and look at all the woodwork which was admittedly pretty, but also meant that I was starting to feel a bit of a lemon standing at the bar with my order pad in hand and grinning like a maniac, so after a minute or so I walked over and asked if I could help.
The first chap was Swiss and spoke to me in Schweizerdeutsch, which fortunately I could understand and speak enough to get by as a waitress.
He asked me if it was alright for them to sit outside on the terrace, and although I wanted to say 'No you bastard, I hate carrying things up and down those bloody steps!' I smiled once more, and replied that it would be fine. To my surprise, he then said would I like to take the orders for drinks then and there to save me the stairs, and warming to him a bit, I said 'certainly!'
He ordered a beer, and then turned to the other two blokes and in English said 'We're going to sit outside but we're placing orders here, so what do you want to drink?'
My heart soared at the thought of the first conversation in which I could understand everything in a long time, and the first 'English' bloke replied with an American accent, 'Do they have beer here?' The Swiss chap said 'Of course they do' and I wrote the order on my pad with the very quick reply of 'Yep, got that, and for you?' in English, to the third man, at which the Swiss chap looked at me, realizing I was English.
The third man who was also American looked indecisive for a moment. After the proverbial penny dropped, he turned to me and said VERY SLOWLY AND LOUDLY, 'I would like a red grape juice please!'
I nicely, and might I add without laughing or use of pointy things, replied 'Yes, one red grape juice on it's way'
I thought that was it and turned to go get the drinks, but was halted by Mr USA #3 grabbing my apron and saying 'I really do want GRAPE JUICE! NOT RED WINE!'
The thought that ran through my head at that point were along, either the man is a) stupid, b) a complete tit, or c) both.
The Swiss chap saw my expression and smiled.
I, hating anybody talking down to me replied slowly, calmly and coherently, and above all in English, that I rest assured knew the difference between red wine and grape juice, and would honestly bring him the latter.
This assurance did not satisfy him.
He again pointed out the fact he wanted GRAPE JUICE, NOT WINE!, and to hammer home the lesson pointed at his 'Fruit of the Loom' T-Shirt that had grapes on it...
I was so tempted to wee in his cup...
By this time, the Swiss chap and the other American bloke had figured out that I was indeed a native English speaker who had wandered abroad, and they were trying to not laugh. Desperately trying. Guffaws were leaking L, R & C.
Anyhoo. The three of them went back outside, and I busied myself getting the drinks together before taking the tray downstairs.
I gave the Swiss chap his beer, he said 'danke freundlich'
I gave American #2 his beer. He said 'Cheers doll'.
I gave fuckwit American #3 his red grape juice...
He took a sip, smiled and said 'THANK YOU!,YOU SPEAK VERY GOOD ENGLISH!'
To which I replied (and to the delight of the Swiss chap and other American), 'Why Thank you! Hardly fu*king surprising though as you're one of the sods who bastardized my language in the first place..'
The bloke could not pluck up courage to speak for the duration of his stay, but his mates had a fantastic time, as did I.
*Mwahahahahaha*

PS. Best conversation I had with an American about 'the common language' was
Me: It's our language, we started it!
Her: Yeah-huh! And we're going to finish it, baby!

Every admiration for honesty there!