chore(09-01): add i18n infrastructure and fix text domain

- Update text domain from 'umzugsliste' to 'siegel-umzugsliste' in plugin header
- Add siegel_umzugsliste_load_textdomain() function hooked on init priority 1
- Add change_locale hook to reload text domain (workaround for WP core bug #39210)
- Text domain loading enables translation file support in languages/ directory

umzugsliste.php

@@ -4,7 +4,7 @@
  * Description: Email-basiertes Möbelauswahlsystem für Siegel Umzüge
  * Version: 1.0.0
  * Author: Siegel Umzüge
- * Text Domain: umzugsliste
+ * Text Domain: siegel-umzugsliste
  * Domain Path: /languages
  */
 
@@ -18,6 +18,31 @@ define( 'UMZUGSLISTE_VERSION', '1.0.0' );
 define( 'UMZUGSLISTE_PLUGIN_DIR', plugin_dir_path( __FILE__ ) );
 define( 'UMZUGSLISTE_PLUGIN_URL', plugin_dir_url( __FILE__ ) );
 
+/**
+ * Load plugin text domain for translations
+ */
+function siegel_umzugsliste_load_textdomain() {
+	load_plugin_textdomain(
+		'siegel-umzugsliste',
+		false,
+		dirname( plugin_basename( __FILE__ ) ) . '/languages'
+	);
+}
+add_action( 'init', 'siegel_umzugsliste_load_textdomain', 1 );
+
+/**
+ * Reload text domain on locale change
+ * Workaround for WordPress core bug #39210 where switch_to_locale() doesn't reload plugin translations
+ */
+add_action( 'change_locale', function() {
+	unload_textdomain( 'siegel-umzugsliste' );
+	load_plugin_textdomain(
+		'siegel-umzugsliste',
+		false,
+		dirname( plugin_basename( __FILE__ ) ) . '/languages'
+	);
+} );
+
 /**
  * Main plugin class
  */