Here are a few steps to consider.  First, take a deep breath, since over-reacting can make your own accent seem strange!  Then try to ask the person to repeat themselves, to be sure you heard them correctly.  Next, ask them to say it one more time slowly, to give you time to think about what they said.  Then say something simple to let them know you have understood.  Then ask them to say it again, this time speaking a little more slowly.  The idea here is to make sure you both communicate well together.
assistant
You can try to speak slowly and clearly, for example, or wait for a pause and then speak. Or you can ask them to repeat what they’ve said.
assistant
What do you do if a foreigner has a strange accent and you cannot understand?
user