Sooner or later, encoded messages became publicly available anyway. However, the kind of semi-confidential gossip that distinguishes the cable traffic released by Wikileaks was often not considered important enough to encrypt or keep confidential.
For example, the Dutch newsweekly Vrij Nederland noted in its December 4th issue (gated, sorry) that John Adams, on a tour of Europe, wrote to Abigail on Sept. 15, 1780 offering some opinions on the Netherlands. Vrij Nederland displays a facsimile of the letter, which unfortunately I could not find reproduced online. Amusingly, the Pieter Groet, the author of the accompanying note, has trouble with Adams' handwriting, and believes that Adams called the Netherlands "the greatest country in the world." Adams' handwriting is quite legible, however, and it is quite clear that he considered it "the greatest curiosity in the world" (emphasis mine).
Adams' complete assessment reads:
"The country where I am is the greatest curiosity in the world. This nation is not known any where, not even by its neighbors. The Dutch language is spoken by none but themselves. Therefore they converse with nobody, and nobody converses with them. The English are a great nation, and they despise the Dutch because they are smaller. The French are a greater nation still, and therefore they despise the Dutch because they are still smaller in comparison to them. But I doubt much whether there is any nation of Europe more estimable than the Dutch in proportion. Their industry and economy ought to be examples to the world. They have less ambition, I mean that of conquest and military glory, than their neighbors, but I don't perceive that they have more avarice. And they carry learning and arts, I think, to greater extent."On balance, Adams was pretty positive — a good thing, since he would become the U.S.'s first official ambassador ('minister plenipotentiary', actually) to the Netherlands two years later. The quip about language is only partially correct: as continues to be true today, almost nobody but the Dutch speaks Dutch.
But the Dutch have always reacted to this by learning multiple languages, even back then, making it possible for them to converse with more people, not fewer. Indeed, Dutch poets of the age revelled in writing poems with words or phrases in multiple languages (Constantijn Huygens' "Olla Podrida" contains 8 languages: Dutch, English, French, German, Italian, Spanish, Latin, and Greek).