Manual:MergeMessageFileList.php/en

Details
mergeMessageFileList.php file is a maintenance script to merge from various extensions to produce a single array containing all message files.

Usage
Output: masterMessagesFile.php

Common error

 * Failed to write to $outputFile

It comes when the user running who the script does not have permission to write the file.

If you are root user then you can run script with sudo.