Manual:$wgExtensionMessagesFiles

This should only be used to specify files that define  and . Its usage for   is deprecated.

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 deprecated; instead, store messages in JSON format and use  . For setting other variables than   (such as  and ), <tvar|4> </> should still be used.

If there is an entry in <tvar|1> </> with the same key as one in <tvar|2> </>, then any <tvar|3> </> variables set in the <tvar|4> </> file will be ignored. This means an extension that only provides messages can be backwards compatible by using both <tvar|1> </> and <tvar|2> </>, and only one of the two will be used depending on what the version of MediaWiki supports.