I'm a Serbian guy with a very strong grasp of (American) English. When traveling across Europe I always end up feeling ridiculous the moment I open my mouth to speak English. Some anectodes:
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I often went to Greece with my family as a kid, like a lot of Serbs. The struggle of forcing myself to say "hello" instead of "hi" (which feels more natural to me, but older Greeks don't get it) led me to just start saying "hi" in Greek, and l ended up developing this habit of using "hi" in the local language instead of using the dreaded (for me) textbook English "hello".
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In Italy nobody knows what a "bathroom" is. Toilet? That they understand, unfortunately that word made me feel dirty every time I said it because, unlike "bathroom", you're indirectly saying you need to take a dump as opposed to leaving it ambiguous.
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In Spain a shady guy offering drugs came up to me while I was with a friend. "We're fine, thanks," I instinctively replied. "What?" he asked with a puzzled look. I reiterated while gesturing 'no', "we're fine". He still didn't get it. I just gave up and said "no buy" like a caveman.
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A very old-looking (like 85+) English lady came up to me asking for directions. She had this old-timey kind of English accent and seemed shy in a polite British way, so I ended up sounding overly formal, kind of textbook English, because I was afraid if I started talking the way I 'naturally' do in English, she might miss something and not ask me to clarify or repeat myself out of that shy politeness.
There's more but these are the ones from my most recent trips. Lord, save me from the cringe.