How to enable Greek & English Spell Checking on MacOSX
a) Download the file Mail-Spell-Check-eng-gr.zip and move the files in the ZIP to the following folder: ~/Library/Spelling
b) Go to System Preferences > Keyboard > Text > Spelling and choose “English & Greek”
c) Close (Command-Q) and Reopen any application that was open during the above change.
Notice: The above works for almost all applications on Mac (like Mail, Safari, Chrome, etc.) but not for Microsoft Office that uses its own dictionaries. Microsoft Office 2016 for Mac has built-in spell checking. Older Microsoft Office require you to buy the Neurolingo spell checking plugin for Office.