Maybe more of a linguistics question than a history one but I've always heard that part of the reason the Imperial Rescript on Surrender wasn't immediately understood is because the Emperor addressed it in language that only the court and upper-class families could easily understand. So I'm kind of curious as to how stark the difference actually was, and particularly if the Emperor had to "re-learn" the language in order to engage with the common people postwar.
He spoke perfectly normal modern Japanese (well, with a hoity-toity accent as might be expected.) While he certainly didn't have the "common touch" and didn't like to spend time outside of his palaces, he did have a lifelong interest in marine and invertebrate biology and would correspond with experts in that field, i.e. working scientists who didn't study classical Japanese themselves.
His "rescripts", official proclamations, were written in archaic Japanese. Having official documents be in archaic language is far from a Japanese-only thing. Consider the following text, which is on a document on my wall as I type:
To all who shall see these presents, greeting:
Know Ye, that reposing special trust and confidence in the fidelity and abilities of /u/Stalking_Goat, I do appoint this Marine a Corporal in the United States Marine Corps to rank as such from the First day of June, two thousand and [a depressingly long time ago]. This appointee will therefore carefully and diligently discharge the duties of the grade to which appointed by doing and performing all manner of things thereunto pertaining. And I do strictly charge and require all personnel of lesser grade to render obedience to appropriate orders. And this appointee is to observe and follow such orders and directions as may be given from time to time by Superiors acting according to the rules and articles governing the discipline of the Armed Forces of the United States of America.
That's very archaic English, with odd capitalization, strange word usages, etc. The imperial rescripts were in the same vein. It's easy enough to follow when read, but it might get confusing if being read over an AM radio by someone with a squeaky voice who is not very good at giving speeches, a person like Emperor Hirohito.
(Admission time, I sort of palmed a card above, because that particular promotion warrant, to Corporal, is the most archaic of the bunch. The other promotion warrants are still archaic but not quite so much so.)
As to exactly how different the Japanese used on the rescripts was from the Japanese used in ordinary conversations, that I can't say. You'd need a better linguist that I.
Also, the difficultly that people might have had in understanding the rescript ending the war when read over the radio was anticipated. Immediately after Hirohito's speech was broadcast, it was repeated by a radio announcer named Wada Shinken, who re-cast it in ordinary speech. One suspects that cleared up any remaining confusion.
Source for all the above is Herbert Bix, Hirohito and the Making of Modern Japan. Well, that and MCO 1400.32D, the US Marine Corps Promotions Manual.