Manual:$wgExtensionMessagesFiles/en

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 $wgMessagesDirs with the same key as one in $wgExtensionMessagesFiles, then any  variables set in the $wgExtensionMessagesFiles file will be ignored. This means an extension that only provides messages can be backwards compatible by using both  and , and only one of the two will be used depending on what the version of MediaWiki supports.