Speaking the Same Language

Papua New Guinea is a small island that contains over 800 languages. I am currently learning the trade language that bridges the gap among them - Melanesian Pidgin. I have made many mistakes as I have been learning this language, but all the people who are helping me learn Pidgin are very gracious and patient with me. However, sometimes it is frustrating not to be able to communicate my thoughts, and I seek refuge in my comfort zone, my native tongue - English.
Alas, but even there I am not safe. I have learned many lessons from working with people from all over the globe - Papua New Guinea, Australia, USA, Canada, England, Germany, the Netherlands, Northern Ireland, Ghana, the Philippines,...Texas - and one of these lessons is that just because someone speaks English, doesn't mean we speak the same language. Let me share with you a few examples of what I mean.
Even among Americans, English is a language of variety. Being from the South, I often am given questioning stares after saying some sort of colloquialism. The two most common Southern expressions that come from my mouth are "y'all" and "might could." While most people tend to accept y'all (as they rightly should as the 2nd person plural pronoun), might could is often met with ridicule. Apparently, people from the Pacific Northwest never have any question as to whether they can or can't do something. :-)
Teaching English in such an international setting has its own set of challenges. Commonwealth English, used in countries such as Australia, Canada, and England, have different punctuation and spelling rules than American English, so I have to tweak my grammar lessons accordingly and always know what country my student is from as I am grading his/her papers. Even some of their words are different. In Mississippi a biscuit is something you eat for breakfast and put gravy on, but in Commonwealth countries a biscuit is a cookie. And don't even get me started on all the unique words spoken in Australia: umbrella - brollie, eski - cooler, fair dinkim - that's true/right on, or thong - flip flop (that one can really cause some miscommunication).
All of this came to my mind as I was teaching a P.E. class the other day. The sport we were playing that day was American football. As I was giving out positions, I told one of my students from PNG to be a linesman, expecting him to line up with the ball. But instead he went over and stood on the sideline. So I told him again that he was a linesman, thinking that he had heard me wrong, but he continued to stand on the sideline. Just as I was about to reprimand him for not going to his proper position, another student came to my aid by saying to him, "...not a line judge." Ah, all of a sudden it became clear. To my PNG student, a "linesman" was the official who stood on the sideline and watched for when the ball went across the line and was out of bounds. I quickly explained what a linesman was in American football, all the while being thankful that I had not reprimanded him for simply going to the position that I had told him to go to.