For me, these are what makes language fun and worth learning. I think in Portuguese, English, and Spanish and have trouble translating the idioms from one to the others without laughing. What are the idioms in the languages you speak and how would you best translate them?
One of my favourite Portuguese idioms is "dar a volta por cima". You can translate it as "to overcome [a setback]" if you're in a pinch, but it goes deeper than that: you are not just recovering from a setback, but also ending up ever better than you were before. The sentiment is similar to that expressed by the English proverb "what doesn't kill you makes you stronger".
Giving a literal translation is also kind of difficult. "Dar a volta" is taking a turn or going along a curved path; for example, when you are driving and you have to go around a church to reach a destination behind it, you "deu a volta na/pela igreja" ("went around the church"); it can also mean turning around (in order to go back). "Por cima" can be translated as "over" in some cases, but not always; it means "through the top part of something".
I like the way Spanish says "realize" as "darse cuenta de" which to say "I realize" you literally say "I give myself account of" or "me doy cuenta de...".
Here is one that I don't hear anyone else use outside my family: In answer to the question What are you doing or What have you been up to, you reply "I mildew to keep from molding." It indicates, I think, that you're doing the same old thing, or trying to keep up by acting in a way that's not completely helpful but it's the best you can manage at the time.