MacLinguist can even translate filenames - just point your mouse on a file in Finder! If you prefer only a certain part of text to be translated, just select that part of the text, and press the Control button twice. It can be text in any arbitrary application: a paragraph of text in Safari, some text on a system button or even a menu item in Menu Bar. After you've pressed the Control button twice, MacLinguist reads the text of the element which is currently located under the mouse cursor.
