Native speakers are hard to understand in a lingua franca situation
There is another aspect beyond raw vocabulary and grammar, and that's unspoken cultural assumptions :not abbreviations and slang, but real misunderstandings caused by different understandings of what seems to be a set of perfectly obvious sentences to both sides .A branch of my company went through a merger with another branch in a different country, and with a different language. Needless to say, it was difficult, and people didn't all take it well. The interesting part was that both sides thought the other side was being actively hostile, and were brandishing emails as 'proof'.I was eventually asked to do something to reconcile both sides of the new merged team, since I am a native speaker of both languages and understand both cultures. I read the emails, and was flabbergasted. Side A could read email from B as hostile, but side B had genuinely sent something they thought was perfectly ok, and I could see why. The other way round was the same. To be more specific, some cultures are far more direct than others, and what seems offensive to one is perfectly acceptable to the other. The other way round would be saying something 'forcefully'  but very indirectly, and the other side understanding it as 'no' when it meant 'yes'.I ended up solving the problem by organizing a very nice lunch, where everyone realized the 'others' were actually nice and friendly human beings, and that email was not always the best way to communicate.