There is a bulleted list in the middle of the February 2016 newsletter for VisualEditor. This list includes MediaWiki messages. They automatically display in the language of the user's preferences (if the messages have been translated at Translatewiki.net yet). Compare, for example:
English (original): https://meta.wikimedia.org/wiki/VisualEditor/Newsletter/2016/February
Polish: https://meta.wikimedia.org/wiki/VisualEditor/Newsletter/2016/February?uselang=pl
Japanese: https://meta.wikimedia.org/wiki/VisualEditor/Newsletter/2016/February?uselang=ja
This internationalization feature is not working in Flow. https://pl.wikipedia.org/wiki/Wątek:Sz4s81homyd3i5ss?uselang=en shows the messages in Polish. https://pl.wikipedia.org/wiki/Wątek:Sz4s81homyd3i5ss?uselang=ja shows the messages in Polish. Even https://pl.wikipedia.org/wiki/Wątek:Sz4s81homyd3i5ss?uselang=qqx shows the messages in Polish.
Flow should respect uselang and setlang for message contents.