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I'm not sure if this is a bug or not, but having just added the Ion Auth library and views to a Fuel install (most recent) I've found that the Ion Auth lang files that feature a percent character for string replacement need doubling up to work: eg
$lang['change_password_new_password_label'] = 'New Password (at least %s characters long):';
will throw an error in sprintf() in that library's views, but
$lang['change_password_new_password_label'] = 'New Password (at least %%s characters long):';
will work as intended.
Ion Auth views / Controller uses sprintf() with nested lang() functions, and I think there is an initial parse happening in MY_language_helper which prematurely requires a replacement string, which is only supplied in the view or controller.