What to say if you don't understand a native speaker:

​One of the biggest challenges for professionals speaking English at work is understanding native English speakers in real conversations. 
Maybe it's a call.
Maybe it's a chat in a coffee break.
Maybe it's someone you've just met at a conference.

The native speaker is talking and you can't understand enough. Maybe you are missing some important details or perhaps you are totally confused.

When this happens, many of my clients say they feel embarrassed, ashamed or like they are failing. Normally they try to hide the fact that they don’t understand. 

But you shouldn’t feel this way. Communication is a two-way process and the native speaker also has a responsibility to speak at a level that you can understand. Therefore, it is perfectly OK to ask them to adapt the way they speak slightly and you don't need to apologise for this!

Here are two useful phrases to politely and professionally do this:

If the person is speaking too fast:
“Please could you slow down. English is not my first language, so I need a little time to process what you’re saying.”

If the problem is the language (idioms, technical language, very complex words):
“Please could you use simpler words. English is not my first language, and it is difficult for me to understand everything you're saying.”