(colloquial, idiomatic) to be a crafty, sly devil/fox, to be a sly dog, to be sly as a fox (be crafty, sly, or occasionally impish and punchy in spite of one’s harmless, innocent, naive appearance) (of people; more rarely, of animals) / (colloquial, idiomatic, figuratively) not to be as harmless, innocent, naive as it may seem at first glance; not to be underestimated (of things and abstract notions)

(humorous) Said as a reply to a statement that is out of context, incomprehensible, unnecessarily complicated or detailed, etc.

all that glitters is not gold

Alternative form of nichts Neues unter der Sonne

"a master has never come falling from heaven"; to become good at something, effort is required; don’t rely on providence

one should strike while the iron is hot

Things never turn out quite as bad as they seem at first (or as pessimists/alarmists make them seem).

one is treated the same way as one treats other people; your own treatment of people will fall back on you; what goes around comes around

(humorous) once your reputation is ruined, you can do as you please and enjoy life; ≈ if you've got the name, you might as well have the game / (more negative) once their reputation is ruined, people tend to cast aside all decency

Esperanto