diff --git a/frontend/src/lib/messages.spec.ts b/frontend/src/lib/messages.spec.ts
index 155ae1a3..22f1527b 100644
--- a/frontend/src/lib/messages.spec.ts
+++ b/frontend/src/lib/messages.spec.ts
@@ -133,4 +133,33 @@ describe('message key parity', () => {
 			expect(es, `missing key in es: ${key}`).toHaveProperty(key);
 		}
 	});
+
+	// #827 REQ-012: the grouping toggle + bucket strings are new Paraglide keys in
+	// every locale; the pre-existing timeline_grouping_date / timeline_tag_chip_label /
+	// timeline_filter_* set is reused, never re-added.
+	it('zeitstrahl grouping + bucket keys are present in all locales (#827 REQ-012)', () => {
+		const requiredKeys = [
+			'timeline_grouping_event',
+			'timeline_grouping_thema',
+			'timeline_grouping_aria_label',
+			'timeline_grouping_segment_date',
+			'timeline_grouping_segment_event',
+			'timeline_grouping_segment_thema',
+			'timeline_grouping_segment_date_short',
+			'timeline_grouping_segment_event_short',
+			'timeline_grouping_segment_thema_short',
+			'timeline_grouping_disabled_reason',
+			'timeline_grouping_multitag_hint',
+			'timeline_bucket_other_letters',
+			'timeline_bucket_no_topic'
+		];
+		for (const key of requiredKeys) {
+			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);
+		}
+		// the pre-existing meta-line + chip keys are reused by #827, not re-declared
+		expect(de).toHaveProperty('timeline_grouping_date');
+		expect(de).toHaveProperty('timeline_tag_chip_label');
+	});
 });