能ある鷹は爪を隠す (Nō aru taka wa tsume wo kakusu)

Meaning: Truly able people don't show off their abilities.

Literally: A talented hawk hides its talons

Example: “彼は普段おとなしいが、実はプロ級のピアニストだ。能ある鷹は爪を隠す。” — He's usually quiet, but he's actually a professional-level pianist: still waters run deep.

Equivalents in English, French, Spanish, German, Italian, and 3 more languages

English: Still waters run deep
French: Il n'est pire eau que l'eau qui dort (literally: There is no worse water than sleeping water)
Spanish: Del agua mansa líbreme Dios (literally: God save me from still water)
German: Stille Wasser sind tief (literally: Still waters are deep)
Italian: Acqua cheta rompe i ponti (literally: Still water breaks bridges)
Dutch: Stille waters hebben diepe gronden (literally: Still waters have deep bottoms)
Russian: В тихом омуте черти водятся (V tikhom omute cherti vodyatsya) (literally: Devils live in a still pool)
Polish: Cicha woda brzegi rwie (literally: Quiet water tears at its banks)