LanguageTool requires Java 8 or later, which is available for the Mac only as a 64 bit version. You need the 64-bit version of LibreOffice 4.2 or later in order to use LanguageTool on a Mac and you also need to have 'Java for OS X' installed, additionally to Java 8 (Source: #74877).
Mac-specific issues
Updated over a week ago