1-Revenge is a dish that is best served cold
Which is not really a Klingon proverb, but is from various Earth sources, notably Les Liaisons Dangereuses
. I've never taken Khan's line about it too seriously -- after all, when did he get the chance to learn anything about Klingons?
Another Earth saying that was actually portrayed as a Klingon saying was "Today is a good day to die" -- which has long been attributed to Crazy Horse, but Wikipedia says it's an English bastardization of a Sioux battle cry meaning "I am ready for whatever comes." (The 2006 BBC Robin Hood
TV series made it Little John's catchphrase, predating Crazy Horse by about seven centuries.)
A couple of lines from "Day of the Dove" -- "Four thousand throats can be slit in one night by a running man" and "Only a fool fights in a burning house" -- have been alleged to be Klingon proverbs by later works of tie-in fiction, but I don't buy that in the first case. That line was spoken in response to the line "We are forty against four hundred," so the fact that the speaker used the specific number four thousand suggests to me that he was making it up on the spot, or maybe paraphrasing a saying with a less specific number.