iTranslate supports more than 80 languages and is accessible from the Mac menu bar. This is a very polished app, that displays pronunciations along with voice output so it's actually possible to hear the words in your selected language.
The app allows you to define a keyboard shortcut to launch it, and it supports cut and paste as well as the built-in speech recognition in recent versions of OS X. An anchor icon at the upper left keeps the app window in the foreground.
iTranslate supports different dialects, and male and female voices. You can even control the speaking rate. The app also supports dictionaries for common words and phrases in many of the supported languages.
The app is generally easy to use, but when it launched all I saw was English to German translation. Users have to click on the flags (see images at the top of the post and in the slideshow) to see the list of other options. It wasn't very obvious, but in retrospect I can see why they did this, as it keeps the GUI quite simple. There is detailed help available within iTranslate.
The app requires an internet connection because all the processing is done off your Mac, as would be expected.
The very similar Google Translate is also available for Mac users through any web browser, but I preferred the ease of use of iTranslate. Google's translation capabilities are also accessible through some other apps in the Mac App Store like Quick Translate Pro for $1.99. and Translate Tab for $3.99.
iTranslate requires OS X 10.8 or later and a 64-bit processor. I liked this app; it is simple and powerful, and always a click away. Given the competition to the app, I think the $4.99 price is OK. The usual $10.00 price seems a bit steep given what else is available for free, even though iTranslate has a very deep feature set.