LeoService 1.0.19 for Mac could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The actual developer of this free Mac application is Bjoern Kriews. Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe.
From the developer:
With LeoService, you can highlight a word in most applications and then hit a keyboard shortcut (Command-Zero by default) to open a web page with dict.leo.org. If the engine recognizes the word as being English or German it will show a list of translations.