What should you do when you just cannot understand somebody's accent?

What's the context? Work colleague, boss, social? In any event, if you can't understand someone, then you could ask them to repeat it more slowly, perhaps spell out key words, say what you think they said and ask for confirmation, ... If remote, you have the option of switching to / supplementing with Slack. You can always portray this as a problem you are having rather than a fault of theirs.

"I'm sorry, I'm having difficulty understanding. Will you say it again and slower, please?"

Or write it out, or switch to a different language if you have one in common.