Manual:$wgForceUIMsgAsContentMsg/ja

List of system messages that should be displayed in user's interface language, even if they would ordinarily by displayed in site's content language.

Details
When translating messages with, it is not always clear what should be considered UI messages and what should be content messages.

For example, for regular Wikipedia sites like en.wikipedia.org, there should be only one 'mainpage'; therefore when getting the link of 'mainpage', treat it as content of the site and call  while, for rendering the text of the link, call. The code in default behaves this way. However, multilingual sites like Commons do offer different versions of 'mainpage' and the like for different languages. This array provides a way to override the default behavior. For example, to allow language specific mainpage and community portal, set