Dann said:
No need to. The Japanese and Koreans were both using Kanji (literally "Han words" = Chinese text) for formal written communication then. The words would have been read differently in sound by all 3 parties but the meaning they were meant to convey would have been perfectly clear to all 3.
As Plotinus said, structurally Chinese is very different from Japanese and Korean. Chinese will recognise the basic nouns, adjectives and verbs making up a sentance in written Japanese or Korean as the kanji for them are being used, but that's it, it seems. They'll get no real idea about verb tense, negations, who's adressing whom, etc. unless they pick up some of the grammar of these languages.
(If you're wondering I once spent a year as a student of Japanese, in an East Asian Studies program alongside people studying Chinese and Korean, and the general verdict was that none of these languages is actually intelligible to the others. I just did that year, before the Army got hold of me, and when they were done with my sorry arse, I had aquired new interests and never got back to the Japanese.)
What the Korean and Japanese did early on was write Chinese with supplementary diacritial marks as clues to non-Chinese readers how it should be understood. And later they developed their own writing systems that could hand in particular inflection.
But there certainly was no need to muck about with Sanskrit, as the Koreans and Japanse would have learnt Chinese anyway. Not just the kanji, but the entire grammatical structure of it. I just mentioned it as an outside possibility.
Dann said:
(Thus to nitpick, the ancient Japanese were not writing in Chinese. They were writing in their own language using Kanji, which are identical to Chinese characters.)
Actually they were writing in Chinese. Bad Chinese. The classical Japanese literature of around 1000 AD written in the Japanese was written by women, and men writing under female pseudonyms ("Tosa Nikki").
Why? Because the females lacked the formal education necessary to write in Chinese proper. The men of the court were expected to write Real Literature in The Only Real Literary Language, i.e. Chinese. The females were supposedly just mucking about with the "crude" native Japanese. But it was the females ended up writing the literary masterpieces (and the odd bloke who realised what the only real ball-game in town was).
And IIRC Chinese continued to be the language of choice for Japanese imperial edicts for centuries afterwards (not that the emperor would wield much power admitedly), so things did get written down officially in Chinese in Japan as well for a very long time it seems.
