i18n: add Russian, French and German menu catalogs

Three new 618-entry menu catalogs (ru, fr, de) with the same
(command, English label) keys as the existing six, registered in
CATALOG_CODES and the Settings language registry. Following the
Spanish/Portuguese precedent these are menu-only catalogs: panels keep
English for now.

crates/engine/src/prefs.rs

@@ -32,6 +32,9 @@ pub const LANGUAGES: &[(&str, &str)] = &[
     ("Українська", "uk"),
     ("Español", "es"),
     ("Português (Brasil)", "pt-br"),
+    ("Русский", "ru"),
+    ("Français", "fr"),
+    ("Deutsch", "de"),
 ];
 
 /// Settings ▸ Appearance ▸ UI Scale: (label, `appearance.uiScale` percent).