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.