Add (a) Polylang switcher menu item(s) to any menu.

example-menu-item.css

/*
Example CSS of how I was showing one OR the other, based on brwoser-width
*/

@media (max-width: @grid-float-breakpoint-max) {
	.language-code {
	  display: none;
	}
}

@media (min-width: @grid-float-breakpoint) {
	.language-full-name {
	  display: none;
	}
}

polylang-custom-menu-switcher.php

<?php
/**
 * Automatically adds a Polylang Language switcher to a menu
 *
 * ! Please note that this will actually show the language in shortened form ('En')
 * ! as well as long form ('English')
 *
 * You can choose to show / hide each span with CSS
 *
 * .language-code is the short-form
 * .language-full-name is the long-form
 *
 * In my case, I wanted to show 'En' on desktop sizes and 'English' for my fly-out mobile menu.
 *
 */


/**
 * This hook lets you add html to the end of a menu list (before the closing '</ul>'
 *
 * Reference: https://developer.wordpress.org/reference/hooks/wp_nav_menu_items/
 */
add_filter( 'wp_nav_menu_items', 'iso_add_menu_lang_switcher', 10, 2 );
function iso_add_menu_lang_switcher( $items, $args ) {

	/**
	 * Enter the name of the menu slug you want to add this switcher to
	 *
	 * Chances are that if you look in your theme's function.php or elsewhere in the code,
	 * you'll find a register_nav_menus function.
	 * If you look at the arguments, you should find the slug for menu you want to affect
	 */
	$menu_slug = 'primary';


	/**
	 * Personally, I see no reason to show the current language (For example, "French" if you're on a French page)
	 * If you really want to show it, set this to true;
	 */
	$show_current_language = false;

	if ( $menu_slug === $args->theme_location ) {

		$langs = array();
		if ( function_exists( 'icl_get_languages' ) ) {
			$available_languages = icl_get_languages( 'skip_missing=0' );
			foreach ( $available_languages as $l ) {
				if ( ! $l['active'] || $show_current_language ) {
					//$langs[] = '<a href="' . $l['url'] . '" class="language_switch_link"><span class="language-code">' . $l['language_code'] . '</span><span class="language-full-name">' . $l['native_name'] . '</span></a>';

					/**
					 * Broken into multiple lines to make easier to read / alter
					 */
					$menu_item = '<li class="menu-item language-switcher">';
					$menu_item .= '<a href="' . $l['url'] . '" class="language_switch_link">';
					$menu_item .= '<span class="language-code">' . $l['language_code'] . '</span>';
					$menu_item .= '<span class="language-full-name">' . $l['native_name'] . '</span>';
					$menu_item .= '</a>';
					$menu_item .= '</li>';

					$langs[] = $menu_item;
				}
			}

			$items .= join( '', $langs );
		}
	}

	return $items;
}