[I've removed the link from my "Old examples" page to this one, because it turns out that the Romontsch example I describe below was nothing special -- in The Quote Verifier by Ralph Keyes, quotes are given showing that "Good fences make good neighbors" is "a very old adage", in English as well as other languages. I'll remove this page eventually, but I thought I should put in this explanation for anyone who might have bookmarked it and was interested.]
In the poem Mending Wall, Robert Frost and his neighbor are fixing the stone wall between their properties, from which the Winter has tumbled many of the stones, as the two of them do every Spring. Frost wonders whether there is a point in continually rebuilding a wall that nature repeatedly tears down and which serves no real purpose, but the other insists, "Good fences make good neighbors".
It is not only in the English-speaking world that this saying is well known from Frost's poem. If one translates the last few words into another language, e.g., "hacen buenos vecinos", and searches for it on the web, one regularly finds the same proverb, attributed to Frost. But if the poem is based on a real incident, there must already have been such a proverb. I've finally run into one, in Romontsch, a language spoken by a few thousand people in one canton of Switzerland. A grammar of that language, Bien di, bien onn ©1965, by Sep Modest Nay, written for German-speaking schoolchildren, contains a large number of proverbs, including, on p.76:
Buns tiarms fan buns vischins.
The word "tierm" (pl. tiarms) is translated in the back as "Grenzstein, Markstein", i.e., boundary-stone, while "vischin", like French "voisin", Spanish "vecino" etc., means "neighbor". (The combination "sch" in that word is pronounced "zh", like the "s" in the English word "vision". In Romontsch "sch" can spell either "sh" or "zh"; one can tell that it is "zh" in this case from the fact that in the corresponding French word "voisin", the "s" is pronounced "z". Otherwise, the words in this saying are pronounced as one would expect in a European language.)
So was Frost's neighbor Swiss?