feat: add language switcher to header nav

Uses Congo built-in locale action menu item.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config/_default/languages.de.toml

@@ -18,6 +18,13 @@ contentDir = "content/de"
     { rss = "/index.xml" },
   ]
 
+[[menus.main]]
+  identifier = "locale"
+  name = "EN"
+  weight = 1
+  [menus.main.params]
+    action = "locale"
+
 [[menus.main]]
   identifier = "about"
   name = "Über uns"

config/_default/languages.en.toml

@@ -18,6 +18,13 @@ contentDir = "content/en"
     { rss = "/index.xml" },
   ]
 
+[[menus.main]]
+  identifier = "locale"
+  name = "DE"
+  weight = 1
+  [menus.main.params]
+    action = "locale"
+
 [[menus.main]]
   identifier = "about"
   name = "About"