Currently Parsoid does not recognize interwiki / language links and just treats them as regular wiki links.
The most important part of this for the December release is the recognition of language links, which are normally rendered as 'Languages' in the sidebar.
To this end, the prefix of a wiki link processed in the LinkHandler (js/lib/ext.core.LinkHandler.js) needs to be matched against a regexp of interwiki prefixes. We already have such a regexp available in the environment as env.interwikiRegexp, which should be reachable as this.manager.env.interwikiRegexp inside the link handler.
For recognized interwiki links ([[de:Foo]] for example), no link should be rendered and something like <link rel="mw:WikiLink/Language" href="de:Foo"> should be produced instead. This also needs to be round-tripped, which will require a small tweak in the serializer. Please ask in #mediawiki-parsoid when you get to this stage.