I don't think it's unethical, but I do think it's kind of rude.

If you're looking for idioms, give bab.la a try : http://en.bab.la/dictionary/english-portuguese/

If you're looking to learn/improve your English, take a look at Duolingo : http://www.duolingo.com/ -- it even supports Brazilian Portuguese.

If you really want to understand the links posted on HN and contribute to the discussion, learning some English is obviously the way to go. Make the effort, it's well worth it !

Good luck

(I'm not a native english speaker. I learned by watching US TV series with english subtitles, pausing the video every single word I did not know to look it up on wordreference.com. I eventually didn't need the subtitles anymore.)