In English: Empty vessels make the most noise. -- Those people who have a little knowledge usually talk the most and make the greatest fuss.
In Russian: Пустая бочка пуще гремит
In Polish: Krowa ktora duzo ryczy malo mleka daje. -- The cow which moos a lot gives little milk. = The person who boasts a lot about his talents/possibilities rarely has them or hardly ever does what he promises to.