Because English is spoken across the world, we often forget that natives from other cultures might not necessarily understand what we say.
Why is that? Well English is spoken across the world, however idioms, jargon and slang are not or rarely taught there! So when we come to do business let´s say in Asia or any other country where English is not the mother tongue, we might face misunderstandings!
The same will happen when our counterparts speak with us or write to us ! The probability of us understanding 100% of what they are writing and what they are saying may be slim. It may be slim because we think differently and have different ways of expressing ourselves. Some cultures are direct; hence what they say is what they mean! However others are indirect, thus context has to be taken into account, and if we do not know the context we may be faced with misunderstandings.
The best way to deal with it would be when we address a non native English speaker, speak slowly, use simple English, rephrase if we feel we have not been understood, pause so that our counterpart has time to take in what we said. And from our side, listen carefully, try to understand the context. If we do not understand, ask for clarifications and avoid using slang, jargon or idioms!