diff --git a/frontend/messages/de.json b/frontend/messages/de.json
index 72254cf3..dcd08246 100644
--- a/frontend/messages/de.json
+++ b/frontend/messages/de.json
@@ -28,6 +28,8 @@
 	"nav_conversations": "Briefwechsel",
 	"nav_admin": "Admin",
 	"nav_logout": "Abmelden",
+	"layout_menu_open": "Menü öffnen",
+	"layout_menu_close": "Menü schließen",
 	"theme_toggle_to_light": "Zu hellem Design wechseln",
 	"theme_toggle_to_dark": "Zu dunklem Design wechseln",
 	"btn_save": "Speichern",
@@ -394,6 +396,10 @@
 	"doc_panel_discussion_annotation_tab": "Annotation · Seite {page}",
 	"pdf_annotations_show": "Annotierungen anzeigen",
 	"pdf_annotations_hide": "Annotierungen verbergen",
+	"viewer_previous_page": "Zurück",
+	"viewer_next_page": "Weiter",
+	"viewer_zoom_out": "Verkleinern",
+	"viewer_zoom_in": "Vergrößern",
 	"upload_action": "Hochladen",
 	"upload_drop_hint": "Einzeln oder mehrere Dateien auf einmal hochladen",
 	"upload_accepted_types": "PDF, JPEG, PNG, TIFF",
@@ -655,6 +661,7 @@
 	"transcription_block_segmentation_only": "Nur Segmentierung",
 	"training_chip_kurrent": "Kurrent-Erkennung",
 	"training_chip_segmentation": "Segmentierung",
+	"transcribe_mark_for_training": "Für Training vormerken",
 	"training_col_type": "Typ",
 	"training_type_base": "Basis",
 	"training_type_personalized": "Personalisiert",
diff --git a/frontend/messages/en.json b/frontend/messages/en.json
index b8b6dd68..f915633e 100644
--- a/frontend/messages/en.json
+++ b/frontend/messages/en.json
@@ -28,6 +28,8 @@
 	"nav_conversations": "Letters",
 	"nav_admin": "Admin",
 	"nav_logout": "Sign out",
+	"layout_menu_open": "Open menu",
+	"layout_menu_close": "Close menu",
 	"theme_toggle_to_light": "Switch to light mode",
 	"theme_toggle_to_dark": "Switch to dark mode",
 	"btn_save": "Save",
@@ -394,6 +396,10 @@
 	"doc_panel_discussion_annotation_tab": "Annotation · Page {page}",
 	"pdf_annotations_show": "Show annotations",
 	"pdf_annotations_hide": "Hide annotations",
+	"viewer_previous_page": "Previous page",
+	"viewer_next_page": "Next page",
+	"viewer_zoom_out": "Zoom out",
+	"viewer_zoom_in": "Zoom in",
 	"upload_action": "Upload",
 	"upload_drop_hint": "Drop one or multiple files at once",
 	"upload_accepted_types": "PDF, JPEG, PNG, TIFF",
@@ -655,6 +661,7 @@
 	"transcription_block_segmentation_only": "Segmentation only",
 	"training_chip_kurrent": "Kurrent recognition",
 	"training_chip_segmentation": "Segmentation",
+	"transcribe_mark_for_training": "Mark for OCR training",
 	"training_col_type": "Type",
 	"training_type_base": "Base",
 	"training_type_personalized": "Personalized",
diff --git a/frontend/messages/es.json b/frontend/messages/es.json
index 0943943e..1f1dec02 100644
--- a/frontend/messages/es.json
+++ b/frontend/messages/es.json
@@ -28,6 +28,8 @@
 	"nav_conversations": "Cartas",
 	"nav_admin": "Admin",
 	"nav_logout": "Cerrar sesión",
+	"layout_menu_open": "Abrir menú",
+	"layout_menu_close": "Cerrar menú",
 	"theme_toggle_to_light": "Cambiar a modo claro",
 	"theme_toggle_to_dark": "Cambiar a modo oscuro",
 	"btn_save": "Guardar",
@@ -394,6 +396,10 @@
 	"doc_panel_discussion_annotation_tab": "Anotación · Página {page}",
 	"pdf_annotations_show": "Mostrar anotaciones",
 	"pdf_annotations_hide": "Ocultar anotaciones",
+	"viewer_previous_page": "Página anterior",
+	"viewer_next_page": "Página siguiente",
+	"viewer_zoom_out": "Reducir",
+	"viewer_zoom_in": "Ampliar",
 	"upload_action": "Subir",
 	"upload_drop_hint": "Uno o varios archivos a la vez",
 	"upload_accepted_types": "PDF, JPEG, PNG, TIFF",
@@ -655,6 +661,7 @@
 	"transcription_block_segmentation_only": "Solo segmentación",
 	"training_chip_kurrent": "Reconocimiento Kurrent",
 	"training_chip_segmentation": "Segmentación",
+	"transcribe_mark_for_training": "Marcar para entrenamiento de OCR",
 	"training_col_type": "Tipo",
 	"training_type_base": "Base",
 	"training_type_personalized": "Personalizado",
diff --git a/frontend/src/lib/messages.spec.ts b/frontend/src/lib/messages.spec.ts
new file mode 100644
index 00000000..e6d0b96c
--- /dev/null
+++ b/frontend/src/lib/messages.spec.ts
@@ -0,0 +1,43 @@
+import { describe, it, expect } from 'vitest';
+import de from '../../messages/de.json';
+import en from '../../messages/en.json';
+import es from '../../messages/es.json';
+
+describe('message key parity', () => {
+	it('de, en, and es have identical key sets', () => {
+		const deKeys = Object.keys(de).sort();
+		const enKeys = Object.keys(en).sort();
+		const esKeys = Object.keys(es).sort();
+		expect(enKeys).toEqual(deKeys);
+		expect(esKeys).toEqual(deKeys);
+	});
+
+	it('viewer navigation keys are present in all locales', () => {
+		const requiredViewerKeys = [
+			'viewer_previous_page',
+			'viewer_next_page',
+			'viewer_zoom_out',
+			'viewer_zoom_in'
+		];
+		for (const key of requiredViewerKeys) {
+			expect(de, `missing key in de: ${key}`).toHaveProperty(key);
+			expect(en, `missing key in en: ${key}`).toHaveProperty(key);
+			expect(es, `missing key in es: ${key}`).toHaveProperty(key);
+		}
+	});
+
+	it('transcribe mark-for-training key is present in all locales', () => {
+		expect(de).toHaveProperty('transcribe_mark_for_training');
+		expect(en).toHaveProperty('transcribe_mark_for_training');
+		expect(es).toHaveProperty('transcribe_mark_for_training');
+	});
+
+	it('layout menu open/close keys are present in all locales', () => {
+		expect(de).toHaveProperty('layout_menu_open');
+		expect(de).toHaveProperty('layout_menu_close');
+		expect(en).toHaveProperty('layout_menu_open');
+		expect(en).toHaveProperty('layout_menu_close');
+		expect(es).toHaveProperty('layout_menu_open');
+		expect(es).toHaveProperty('layout_menu_close');
+	});
+});