diff --git a/packages/lib/locale.test.ts b/packages/lib/locale.test.ts
index 0627ef5ba5..641119f3e4 100644
--- a/packages/lib/locale.test.ts
+++ b/packages/lib/locale.test.ts
@@ -1,4 +1,4 @@
-import { closestSupportedLocale, parsePluralForm, setLocale, _n } from './locale';
+import { closestSupportedLocale, parsePluralForm, setLocale, _n, toIso639 } from './locale';
 
 describe('locale', () => {
 
@@ -91,4 +91,14 @@ describe('locale', () => {
 		}
 	});
 
+	test.each([
+		['en_GB', 'eng'],
+		['en', 'eng'],
+		['de', 'deu'],
+		['fr_FR', 'fra'],
+	])('should convert to ISO-639', (input, expected) => {
+		const actual = toIso639(input);
+		expect(actual).toBe(expected);
+	});
+
 });
diff --git a/packages/lib/locale.ts b/packages/lib/locale.ts
index 600a4fed3d..6bf10d6d97 100644
--- a/packages/lib/locale.ts
+++ b/packages/lib/locale.ts
@@ -213,6 +213,7 @@ const iso639Map_ = [
 	['cos', 'co'],
 	['cre', 'cr'],
 	['dan', 'da'],
+	['deu', 'de'],
 	['div', 'dv'],
 	['dzo', 'dz'],
 	['eng', 'en'],