The "language conversion blocks" are a wikitext feature that allow users to define text content in parallel scripts. The most high profile is in Chinese, which has two major writing systems and automated conversion between them, but there are 28 others, some of which are not automated conversion (so VE will need to not just mark the text with an appropriate <span>, but allow the user to understand when they have edited text that needs editing twice or thrice (possibly in scripts they cannot/do not want to use?) - see https://meta.wikimedia.org/wiki/Wikipedias_in_multiple_writing_systems
Documentation of the feature (focussed on the syntax) is here: https://www.mediawiki.org/wiki/Writing_systems/Syntax
Parsoid will need to add support for this first, which is bug 41716.