Thread:Project:Support desk/How to get the selected language (2)/reply

That page contains, and int: is a special magic word that transcludes that message ("lang" in this example) in the current user language.

For this to work, the message should exist, or at least, the relevant MediaWiki: page. This is an artifact, that requires your wiki to have that message in all the languages you want to work with. See meta:Special:PrefixIndex/MediaWiki:Lang. So for it to work, you should create all those pages and subpages.