Before you go out of the country, make a few wallet-sized cards that list what you can and can't eat in the native language(s) of the country you're visiting.

I like to list what I'm able to eat on one side of the card... and list the foods I can't have on the other. This makes it so the server and kitchen can't easily mix them up.

This is especially helpful for vegetarian and vegans, people with religious dietary restrictions, and people with food allergies.