While living in Mt. View, CA, we had a doc from Switzerland visit our lab for a year as a research fellow at Stanford. He and his wife spoke French, German, Italian, and English. He understood English very well, and spoke fluently, albeit with a fairly heavy accent. He did have to learn some idioms and slang, but was otherwise quite articulate.
When we first met, they were ecstatic to find they could understand and communicate with us. The previous night was a layover at Dallas-Fort Worth, and were mortified to discover they could only understand about 30%. They breathed a huge sigh of relief when we laughed and said, "Don't worry, we can't understand 'Texan' either!" After that they did just fine.
I guess that's one of the many things that makes this country so interesting!
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