Manual:$wgExtensionMessagesFiles

Details
Associative array mapping extension name to the filename where messages can be found. The file must create a variable called $messages.

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 $wgMessagesDirs. For setting other variables than $messages, $wgExtensionMessagesFiles should still be used.

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