Using a script with silver searcher (see below) you can find keys that are not referred to in resources or includes folders.
They are the following keys:

• mobile-frontend-pageaction-upload-tooltip
• mobile-frontend-quick-lookup-not-internal
• mobile-frontend-quick-lookup-no-results
• mobile-frontend-quick-lookup-looking
• mobile-frontend-watchlist-view
• mobile-frontend-thanked-notice
• mobile-frontend-sign-up-heading
• mobile-frontend-sign-in-why
• mobile-frontend-sign-in-heading
• mobile-frontend-sign-in-error-heading
• mobile-frontend-search-submit
• mobile-frontend-saving-exit-page
• mobile-frontend-regular-site
• mobile-frontend-placeholder-app
• mobile-frontend-photo-caption-placeholder
• mobile-frontend-photo-article-edit-comment
• mobile-frontend-photo-article-donate-comment
• mobile-frontend-password-placeholder
• mobile-frontend-password-confirm-placeholder
• mobile-frontend-password
• mobile-frontend-page-menu-language-current
• mobile-frontend-page-menu-history
• mobile-frontend-page-menu-contents
• mobile-frontend-low-res-explain
• mobile-frontend-nearby-sectiontext
• mobile-frontend-main-menu-featured
• mobile-frontend-main-menu-account-create
• mobile-frontend-main-menu-back
• mobile-frontend-logged-in-toast-notification
• mobile-frontend-lead-image-tutorial-summary
• mobile-frontend-lead-image-tutorial-confirm
• mobile-frontend-last-modified-years
• mobile-frontend-last-modified-seconds
• mobile-frontend-last-modified-months
• mobile-frontend-last-modified-minutes
• mobile-frontend-last-modified-just-now
• mobile-frontend-last-modified-hours
• mobile-frontend-last-modified-days
• mobile-frontend-image-uploading
• mobile-frontend-image-heading-describe
• mobile-frontend-image-cancel-confirm
• mobile-frontend-footer-contact
• mobile-frontend-first-upload-wizard-new-page-3-ok
• mobile-frontend-first-upload-wizard-new-page-3-header
• mobile-frontend-first-upload-wizard-new-page-2-header
• mobile-frontend-first-upload-wizard-new-page-2
• mobile-frontend-first-upload-wizard-new-page-1-header
• mobile-frontend-first-upload-wizard-new-page-1
• mobile-frontend-enable-images
• mobile-frontend-editor-newpage-prompt
• mobile-frontend-drawer-cancel
• mobile-frontend-donate-image-signup-action
• mobile-frontend-donate-image-heading
• mobile-frontend-dismiss-notification
• mobile-frontend-disable-images
• mobile-frontend-diffview-explained
• mobile-frontend-current-language
• mobile-frontend-contribution-summary
• mobile-frontend-changeslist-ip
• mobile-frontend-back-to-top-of-section
• mobile-frontend-app-title
• mobile-frontend-account-create-submit
• mobile-frontend-account-create-email-placeholder

they should all be removed.

var fs = require('fs');
var exec = require('child_process').exec;
var en = JSON.parse(fs.readFileSync('i18n/en.json', 'utf8'));
Object.keys(en).forEach((key) => {
	var cmd = `ag ${key} includes/ resources/ extension.json`;
	if (
		key.indexOf( 'apihelp-' ) !== 0 &&
		key !== 'skinname-minerva' &&
		key.indexOf( 'tag-' ) !== 0 &&
		key.indexOf( 'abusefilter-edit-builder-vars-' ) !== 0 &&
		key.indexOf('@') !== 0
	) {
		exec(cmd, function(error, stdout, stderr) {
			if ( !stdout && !stderr )
			console.log(`* ${key}`);
		});
	}
} );