This version now requires at least Java 8 (according to official web site). There's also a standalone desktop version provided, which could be used with e.g. Emacs (there's a package for it). Is it possible to provide that via ebuild? Stand-alone desktop version: https://languagetool.org/download/LanguageTool-3.2.zip
Apart from being used by Emacs & co, the standalone version can be used with Texstudio, which is by itself very nice.
On 2016-03-28 version 3.3 was released.
Version 3.4 is now available: https://www.languagetool.org/download/LanguageTool-3.4.oxt
Office extension is bumped
The standalone version was packaged by Radhermit a short while ago as app-text/languagetool. I have just committed 3.5-r1 with some improvements.