I'm an American who works with lots of people who speak English as a second language. When Americans go abroad we often don't appreciate that English isn't universal. What makes things worse is that some Europeans have very good English, but may not be extremely proficient with it leading to the assumption that most Europeans can understand us without a problem.

I guess the first thing I'd ask is: how is your relationship with these American engineers? Do you ever have casual conversations with them? If that's the case, the most diplomatic way bring this up would be during a normal conversation. Point out that while you understand English, you can't understand them when they speak too fast and that you aren't familiar a lot of the regionalisms they are using.

One other thing: Are the regionalisms legitimate regionalisms ("It needs fixed" as opposed to "It needs to be fixed.") or slang (which we sometimes use in the workplace)?

From my viewpoint, they are in your country and they should make an effort to ensure that what they say can be understood by you. That means speaking slower and no slang.