Слово — не воробей, вылетит — не поймаешь (slóvo — ne vorobéi, výletit — ne poimáesh).
Literally: a spoken word is not a sparrow, once it flies out – it cannot be caught.
Meaning: mind what you say to be polite and not to offend someone.
- a word spoken is past recalling;
- when the word is out it belongs to another.
Язык мой — враг мой (yazýk moy — vrag moy).
Literally: my tongue is my enemy.
Explanation: sometimes you may regret what you said even though you didn’t mean anything wrong. So watch what you say because you can hurt people with your words, telling confidential information about another person, etc.
English equivalent: a man’s ruin lies in his tongue.
P.S. This is a classic Soviet poster (1941) saying literally:
In those days
Even walls overhear.
Gossip and idle talk are not far
There are many (de)motivational posters and collages made using it, e.g. one with Snowden: