Quick Dashboard Language Switch in the Admin Bar

add_action( 'admin_bar_menu', function ( $bar ) {
    if ( ! is_admin() || ! is_user_logged_in() ) {
        return;
    }

    $languages = get_available_languages();

    if ( ! $languages ) {
        return;
    }

    array_unshift( $languages, 'en_US' );
    $current = get_user_locale();

    $bar->add_node( array(
        'id'    => 'wpide-admin-lang',
        'title' => esc_html( strtoupper( substr( $current, 0, 2 ) ) ),
        'meta'  => array( 'title' => __( 'Dashboard language' ) ),
    ) );

    foreach ( array_unique( $languages ) as $locale ) {
        $bar->add_node( array(
            'parent' => 'wpide-admin-lang',
            'id'     => 'wpide-admin-lang-' . sanitize_key( $locale ),
            'title'  => esc_html( ( $locale === $current ? '✓ ' : '' ) . $locale ),
            'href'   => wp_nonce_url( add_query_arg( array( 'action' => 'wpide_set_admin_lang', 'locale' => $locale ), admin_url( 'admin-post.php' ) ), 'wpide_set_admin_lang' ),
        ) );
    }
}, 80 );

add_action( 'admin_post_wpide_set_admin_lang', function () {
    check_admin_referer( 'wpide_set_admin_lang' );

    $locale = isset( $_GET['locale'] ) ? sanitize_text_field( wp_unslash( $_GET['locale'] ) ) : '';

    if ( 'en_US' === $locale || in_array( $locale, get_available_languages(), true ) ) {
        update_user_meta( get_current_user_id(), 'locale', 'en_US' === $locale ? 'en_US' : $locale );
    }

    wp_safe_redirect( wp_get_referer() ? wp_get_referer() : admin_url() );
    exit;
} );