Thanks for taking the time to report this.
What is {{int:lang}}? Where can I find more about it? Where can it be used already? Why is it "usable"?
For more information, see https://www.mediawiki.org/wiki/How_to_report_a_bug

It's a magic word that translates system messages.

@kaldari What setting or extension makes int:lang available?

@Dereckson, @Aklapper:
Sorry for small amount of information. This magicword callback language of ui. All WikiMedia projects have This magicword, but {{int:lang}} needs
this pages for work. I think will be better if this pages be in wiki-project by default.

{{int:lang}} is a hack that has only really been used on multilingual wikis. Can you explain why you need it on all wikis?

@Legoktm:
{{int:lang}} can help with templates' or forum/help messages in small-wiki or new language wikiprojects.