'title' => 'ACP_EXTENSIONS_MANAGEMENT',
shows as is in ACP -> Customise.
There is an ACP_EXTENSION_MANAGEMENT language string.
database_update.php correctly updates to ACP_EXTENSION_MANAGEMENT, install_install.php should do so as well.
It appears that this was simply a typo. We need to pick either ACP_EXTENSIONS_MANAGEMENT or ACP_EXTENSION_MANAGEMENT (i.e. with or without the S) and stick with it. I vote we make "extensions" singular across the board.
Patch in progress.