I'm learning English for many years, and I use English at work. Some clients I cannot understand. I ask them to speak slowly. They do, but then they speak quickly again. My bigger problem is talking to French or Indian clients. Did you face this situation and find a solution?
If your question is, "What is the correct English you should use to ask someone to speak more slowly?" Then the problem is that the English is easy, but the * politics* is difficult.
"Please speak slowly and clearly, I am not a native English speaker and I want to make sure I correctly understand you."
Everything after that is politics.