The file: language\en\acp\extensions.php use on a few location structures which are meant for plural rules e.g.:

	'EXTENSION_UPDATING_EXPLAIN'	=> [

		0 => 'Disable the extension',

		1 => 'Delete the extension’s files from the filesystem',

		2 => 'Upload the new files',

		3 => 'Enable the extension',

	],

But this is meant to be used just for plural rules and so give more options for different translations, e.g.:
\language\en\viewforum.php:

	'VIEW_FORUM_TOPICS'		=> array(

		1	=> '%d topic',

		2	=> '%d topics',

	),

This should be changed to an own lang key for each or combined into one.