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 backward 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.