Manual:$wgExtensionMessagesFiles/en

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 $1. For setting other variables than $1 (such as $2 and $3, $4 should still be used.

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