If this change is intended to load all extension lang files at user_setup, it will be problematic.
Language files are already easily added to an extension by simply adding
to a function listening to core.user_setup in an event listener, or you can do it as needed inside your extension php controller files.
This system work, and is better than your global auto-loading proposal for the following reason:
I have extensions with gigantic ACP language files, and no needed lang files for outside the ACP. In addition, stand-alone controller files (like a blog ext) may only need their lang files when they are loaded, but never anywhere else in the forum. This PR would load these extra language files all the time, even though they may only be needed in the ACP or in the standalone file. Also, permission language files, which are separate files, are already being loaded too. So this PR would really just load everything, all the time, even when not needed, and prevent authors from controlling which of their lang files get loaded when/where they want, and the end result is just heaping on more system memory usage on an already bloated and memory heavy phpBB.