when the cat's away. . .

. . . rodents will frolic, in many different ways according to their origin. Here's some cultural variants on this proverb:

The cat is absent and the mice dance. (Modern Greek).
When the cat is gone the mice dance. (Belgian).
When the cat is away the mice have room to play. (Welsh).
When the cat is not in the house the rats (or mice) dance. (Italian).
When the cat is not the mice are awake. (French).
When the cat's away it is jubilee with the mice. (Dutch).
When the cat's away the mice give a ball. (Martinique Creole).
When the cat shall leave home the mice shall have leave to dance. (Irish-Farney). When the cats leave town (or home) the mice dance. (Irish-Ulster).
When the cat sleeps the mice play. (Dutch).
When there is no cat mice dance. (Indian-Kumaun, Garhwal).

But wait! Here's some related proverbs. . .

A blate (bashful, timid - Curator) cat makes a proud mouse. (Scotch).
A blind cat catches only a dead rat. (Chinese).
The mewing of the cat has silenced the mice. (Modern Greek).
Well knows the mouse that the cat's out of the house. (Scotch).
Were the cat at home it were worse for you. (Welsh, Irish).
What wots the mouse, the cat's out of the house. (Scotch).
When the cat dies the mice rejoice. (Ashanti, Oji-West African).
When the cat is blind the rat becomes bold. (Marathi).
When the cat is safe in the forest the rat says—"She's my wife." (Hindustani).

- from The Antiquity of Proverbs: Fifty Familiar Proverbs and Folk Sayings with Annotations and Lists of Connected Forms Found in All Parts of the World, by Dwight Edwards Marvin (New York and London: G. P. Putnam's Sons, 1922), p. 301.