When you install the script, it places a language file in /languages/ -- if an english-only install it copies /languages/setup/englishonly.lng to /languages/default.lng, if it's a multilingual install it copies /languages/setup/fullenglish.lng to /languages/English.lng.

In some cases, people upgrading from ancient versions may somehow have gotten along without this file before the 4.1 series -- the lack of it would prevent certain things from working all along, but didn't die to the error message until 4.1. Some ancient 3.x era installs may have failed to warn that they were unable to create the files due to permissions problems, and this problem could've gone unnoticed until now. If this is the case for you, copy over the applicable file for the type of installation you have: /languages/setup/englishonly.lng to /languages/default.lng or /languages/setup/fullenglish.lng to /languages/English.lng as explained in the previous paragraph.

Also check in /templates/ to be sure you only have either /templates/multilingual/ or /templates/default/ -- not both. If you have both, delete the one you aren't using.