Il troppo stroppia

Meaning: Too much of anything, even a good thing, does harm.

Literally: Too much cripples

Example: “Un cucchiaio di zucchero va bene, tre no: il troppo stroppia.” — One spoon of sugar is fine, three isn't: you can have too much of a good thing.

Equivalents in English, French, Spanish, Japanese, German, and 2 more languages

English: Too much liberty spoils all.
French: Trop de zèle gâte tout (literally: Too much zeal spoils all)
Spanish: Ni tanto ni tan calvo (literally: Neither so much nor so bald (avoid both extremes))
Japanese: 過ぎたるは猶及ばざるが如し (Sugitaru wa nao oyobazaru ga gotoshi) (literally: Going too far is just like falling short)
German: Allzu viel ist ungesund (literally: Too much is unhealthy)
Polish: Co za dużo, to niezdrowo (literally: Too much is unhealthy)
Chinese: 过犹不及 (Guò yóu bù jí) (literally: Going too far is as bad as falling short)