The 'languageshtml' result property for action=parse has been deprecated for some time now, and with the merge of Gerrit change 143299[1] is now removed entirely. This will be deployed to WMF wikis with 1.24wmf12; see https://www.mediawiki.org/wiki/MediaWiki_1.24/Roadmap for the schedule.

[1]: https://gerrit.wikimedia.org/r/#/c/143299/