If Japanese coworkers could not understand your coworker, is it that he used complicated sentences or that he couldn't use them correctly? Being able to explain things in simple terms is really useful, but it also means that you have to explain a lot more when conveying nuance or precision because you can't choose the word that means exactly what you want to say. Do you ever run into this, or do you feel the simple explanations can cover everything you want to communicate?

Do you think there is any benefit from learning idioms? When speaking a non-native language, I always struggle with wanting to sound like a native speaker, and never using idioms and jargon makes me feel like I'm setting myself apart. However, it's really hard to use them correctly when you don't speak the language natively.