The title of my question is a Russian proverb, for which I cannot think of an analog. All the examples I have seen on this website refer to actions rather than specifically speech. Can anyone give me an example of a colloquial phrase about the irreversibility of speech?
Probably not an established set phrase, but I often hear and read:
Words once spoken ( cannot be retrieved, cannot be taken back)
It may derive from the Latin proverb:
Translation: "A word once spoken can never be recalled." From Horace. Another interpretation: "Think twice before you speak."
The bell, once rung, cannot be unrung.
You cannot unring the bell.
Google books traces "cannot be unrung" to 1924:
... what is learned or suspected outside of court may have some influence on the judicial decision. It may be only a subtle or even subconscious influence, but a bell cannot be unrung. Adverse claimants have at least some reason to fear ...
By 1948 it is in the Utah bar bulletin:
if the matter has already been printed and in the hands of the jury, the bell cannot be unrung
by 1956, it was being used as a commonplace in Sandez v US:
Could the court "unring the bell" by subsequently instructing the jury that Exhibit 29 was admissible only against Perno? We think it doubtful.
I also concur with "Let the cat out of the bag". This paints a different word-picture but the sense is similar.
Little said is soonest mended.
George Wither (1588-1667)
The always thorough Ken Greenwood, at Wordwizard, adds this research:
LEAST SAID, SOONEST MENDED proverb: ... The expression dates from the 18th century in this form, but the notion dates from the 15th century (see quote below) and where it appeared in the form, which is still seen, little said (is) soon amended (see 1555 quote below).
(Allen’s English Phrases, Facts on File Encyclopedia of Word and Phrase Origins, Random House Dictionary of America’s Popular Proverbs and Sayings)
letting the cat out of the bag also speaks of the Pandora's Box effect, the impossibility of unscrambling scrambled eggs, here the futility of trying to 'unsay' something, but is used only when a secret has been blabbed.
A lie can be halfway around the world before the truth has got its boots on.
You see this on twitter a lot! The initial tweet might have 10,000 retweets, but the retraction "looks like that wasn't true" will have 100.
This also recalls a Chinese proverb:
A word once spoken, an army of chariots cannot overtake it
Question: What's the context? You could be casual, "once it's out there, it's out there." There was another post about arrows and regrettable utterances... I liked this take on it: "Words can be like arrows shot from a bow, piercing and wounding. Be careful with words, they cannot be unsaid."
I like the response from user: NVZ (with my take on it): "A weapon that has left your hand, a word that has left your mouth - cannot be gotten back."
I checked out this site: https://en.wikiquote.org/wiki/English_proverbs_(alphabetically_by_proverb). The closest proverb I found is this:
"A word spoken is past recalling." Mieder, Wolfgang; Kingsbury, Stewart A.; Harder, Kelsie B. (1992). A Dictionary of American proverbs. p. 925.
Eloquent, I think. I had no idea who Mieder was. Apparently, he's a professor of German and folklore at the University of Vermont (wikipedia).
Hope that helps.