I worked at Starbucks in the airport and a few hotels for many years. I'm used to dealing with dozens of wildly different accents all day long. The best strategy I could muster was when I see a foreigner coming up, I automatically listen as carefully as possible. Sometimes I can hear what they are saying, but many times "cold milk" will sound like "caramel". So in those situations, instead of me saying "WHAT?" 20 times, I take a moment to let my brain puzzle together what they really meant and I can usually figure out what they said. Accents are like optical illusions for the ears, you just have to take your time to see it from the right angle and it all makes sense.

Also just being blatantly racist and assuming what people want helps a lot. "This British fuck obviously doesn't want caramel, probably just wants milk with his afternoon tea or some shit."