mediawiki-skins-Vector/resources/skins.vector.js/languageButton.js

/**
 * Copies interwiki links to main menu
 *
 * Temporary solution to T287206, can be removed when the new ULS built in Vue.js
 * has been released and contains this
 */
function addInterwikiLinkToMainMenu() {
	const editLink = document.querySelector( '#p-lang-btn .wbc-editpage' );

	if ( !editLink ) {
		return;
	}
	const title = editLink.getAttribute( 'title' ) || '';

	const addInterlanguageLink = mw.util.addPortletLink(
		'p-tb',
		editLink.getAttribute( 'href' ) || '#',
		// Original text is "Edit links".
		// Since its taken out of context the title is more descriptive.
		title,
		'wbc-editpage',
		title
	);

	if ( addInterlanguageLink ) {
		addInterlanguageLink.addEventListener( 'click', function ( /** @type {Event} */ e ) {
			e.preventDefault();
			// redirect to the detached and original edit link
			editLink.dispatchEvent( new Event( 'click' ) );
		} );
	}
}

/**
 * Initialize the language button.
 */
module.exports = function () {
	addInterwikiLinkToMainMenu();
};