Ultimately people come to read Manga because they want to be exposed to a new culture, with its own idiots, mannerisms, and ideas. Once you start swapping out clear cultural indicators for some crude equivalent in another language, you've opened the door to rewriting the entire story and denying the readers the bit of culture clash they came for; not only do you fail to capture the full meaning of what's being said, but you take away something the audience specifically came for. I'd like to think that when you see something a bit weird in your stories, you stop to remember that these are fundamentally different people with their own unique background and context.
Now, there are ways to handle it, and they can actually work out really well (Spoilers: Chapter 22 of Pocha Musume). But ultimately, the author already decided precisely what he wanted the characters to say, how they say it, and the audience are interested in hearing/seeing that as closely as is possible, considering the barriers that exist between two foreign nations.
TL;DR: If they say a thing, I'm obliged to leave it in. If we have a way or saying what they say (e.g. 本末転倒=Putting the cart before the horse), then neat. If not, you're going to have to remember it's Japan, and hope I can help you understand.