Not everyone speaks English as their first language.

Welcome to the world.

It’s a diverse place out here.

You will probably have to speak to someone eventually that doesn’t speak perfect {name your country as an adjective here} English.

You may have to listen closely.  You may have to ask politely for that person to repeat themselves.

This is all understandable.

However, it is unacceptable to angrily raise your voice in frustration because the other person doesn’t speak the same brand of English you do.

Of course, maybe you’re not raising your voice in frustration.  Maybe you’re just raising your voice to be understood better.  Let me point two things out:  First, the other person doesn’t have a hearing issue.  Second, you’re the one having difficulty understanding.