fix: step navigation back-click bug and add shortcode lang attribute

Track highestStep so navigating backward preserves completed dots and
allows clicking forward to any previously visited step. Add [umzugsliste
lang="de|en"] shortcode attribute that switches locale via
switch_to_locale() for per-page language control.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

includes/class-shortcode.php

@@ -51,11 +51,26 @@ class Umzugsliste_Shortcode {
 	 * @return string Form HTML
 	 */
 	public function render_form( $atts ) {
-		// Ensure assets are enqueued
-		$this->enqueue_assets();
+		$atts = shortcode_atts( array( 'lang' => '' ), $atts, 'umzugsliste' );
+		$switched = false;
 
-		// Render the form
-		return Umzugsliste_Form_Renderer::render();
+		if ( ! empty( $atts['lang'] ) ) {
+			$locale_map = array( 'de' => 'de_DE', 'en' => 'en_US' );
+			$locale     = isset( $locale_map[ $atts['lang'] ] ) ? $locale_map[ $atts['lang'] ] : '';
+			if ( $locale && $locale !== get_locale() ) {
+				switch_to_locale( $locale );
+				$switched = true;
+			}
+		}
+
+		$this->enqueue_assets();
+		$html = Umzugsliste_Form_Renderer::render();
+
+		if ( $switched ) {
+			restore_previous_locale();
+		}
+
+		return $html;
 	}
 
 	/**