In Mac OS X, it is easy to display the definition of a word. Simply mark a word, right-click on it and select „Look Up” or press ^⌘D. Unfortunately, a good French dictionary is missing, even if you have the French language package installed.
Fortunately, it is possible to add dictionary plugins to the Dictionary application. There used to be a good French dictionary, provided by etresoft.com, available in the public domain, but unfortunately all the relevant download links, such as this one, seem to be broken for quite some time.
However, after doing an extensive search, I managed to retrieve the package from the depths of the web. I am relinking to the file, in case someone else should need it, too. Simply download, extract and install. ◻︎