wfLoadExtensionMessages() is a deprecated MediaWiki function. It was a no-op since 1.16, and was removed entirely in 1.21.
If you are upgrading a wiki that uses old extensions, you may see an error such as:
Fatal error: Call to undefined function wfLoadExtensionMessages() in /w/extensions/Example/Example.php on line 50
Fortunately, since it was a no-op, you can just comment it out of your extension.