My take on this is to speak regular English with a slight foreign accent, which sounds ridiculous, but it actually does seem to be effective.
Incidentally, I am from the North of England but have lived in the South for the past ten years and people sometimes struggle to understand what I am saying. Repeating myself has become the norm.
Then other students came from other countries and then I figured I just didn’t understand that particular accent.
She pronounced where as “wa” or something like that I was really confused.
Indeed, Northern English is just regular English with random vowels and syllables missed out.
It does sound ridiculous, but I do exactly this when speaking to groups of foreign colleagues. I'll slow down a bit, change my consonants and vowels ever so slightly, and use slightly "incorrect" or simplified grammar. And I think it's helped me. At least, it hasn't hindered me.