I don't think international diplomacy is ever going to be President Trump's strong suit. And meeting with the Japanese is probably the best example out there. Japanese culture has elevated good manners to a preposterously high level, to the point where the entire culture would prefer to lie 100% of the time than say somehting out of place. Trump is the opposite of that. He's an Al Cervik like character. Loud, brash, unapologetic, and in a certain respect, base.
None of those things is a negative precisely. I've seen Trump in a more soft spoken setting and I know he can keep from tripping over his feet in the worst way. And there is a great deal to be said for being genuine. Most intelligent people will let it slide if it's clear that you mean well but stumble over decorum in some small way. It's only low cunning and intentional rudeness which are a real problem. Cultural differences are cultural differences. Most of us will make allowances in the right spirit.
Trump is nothing if not genuine, so even if he says or does something which might give offense coming from others (the former condescender in chief comes immediately to mind), I think the bar is very low for him. He's not going out there insulting Abe's ancestry or demanding that he kneel before him. He's just representing the interests of his country in his own unique way. Like Al Cervik, picture above, he may be a bit ... undiplomatic, but it's hard to dislike him for it.
Real leaders don't need to be diplomatic, they just need to avoid condescension and the appearance of arrogance. I'm completely confident Trump can handle that.