Manual:$wgExtensionMessagesFiles

Details
Associative array mapping extension name to the filename where messages can be found.

The file must create a variable called  .

Example:

Changed behavior in MediaWiki 1.23
Since MediaWiki 1.23, use of this variable to define messages is discouraged; instead, store messages in JSON format and use  .

For setting other variables than  ,   should still be used.

If there is an entry in   with the same key as one in  , then any   variables set in the   file will be ignored.

This means an extension that only provides messages can be backwards compatible by using both <tvar|ExtensionMessagesFiles> </> and <tvar|MessagesDirs> </>, and only one of the two will be used depending on what the version of MediaWiki supports.