Thread:Project:Support desk/Change fallback language/reply (3)

You would have to modify the file languages/messages/Language.php. Look for the line. It's probably near the begining.

Note, modifying language files in this manner is unsupported, and will probably break (be reset to the normal default) when you upgrade mediawiki. (I'm not sure if the localization cache will automatically clear itself. I think it does, but if it doesn't you may have to run the rebuildLocalisationCache.php maintinance script)

(You probably already know this, but in the interest of completeness) If there's a specific message you want to change the value of, you can edit it by going to the page Special:allmessages.