diff --git a/lib/registry.ts b/lib/registry.ts
index 979d1228..1c7c5c41 100644
--- a/lib/registry.ts
+++ b/lib/registry.ts
@@ -36,6 +36,12 @@ export function registerAppTranslations(
 	translations: Translations,
 	pluralFunction: PluralFunction
 ) {
+	if (window._oc_l10n_registry_translations === undefined) {
+		window._oc_l10n_registry_translations = {}
+	}
+	if (window._oc_l10n_registry_plural_functions === undefined) {
+		window._oc_l10n_registry_plural_functions = {}
+	}
 	if (!hasAppTranslations(appId)) {
 		setAppTranslations(appId, translations, pluralFunction)
 	} else {
@@ -61,19 +67,14 @@ export function unregisterAppTranslations(appId: string) {
  */
 export function getAppTranslations(appId: string): AppTranslations {
 	if (
-		typeof window._oc_l10n_registry_translations === 'undefined'
-		|| typeof window._oc_l10n_registry_plural_functions === 'undefined'
+		typeof window._oc_l10n_registry_translations?.[appId] === 'undefined'
+		|| typeof window._oc_l10n_registry_plural_functions?.[appId] === 'undefined'
 	) {
-		console.warn('No OC L10N registry found')
-		return {
-			translations: {},
-			pluralFunction: (number: number) => number,
-		}
+		console.warn(`No translation for appId "${appId}" have been registered`)
 	}
-
 	return {
-		translations: window._oc_l10n_registry_translations[appId] || {},
-		pluralFunction: window._oc_l10n_registry_plural_functions[appId],
+		translations: window._oc_l10n_registry_translations?.[appId] ?? {},
+		pluralFunction: window._oc_l10n_registry_plural_functions?.[appId] ?? ((number: number) => number),
 	}
 }
 
diff --git a/tests/index.test.js b/tests/index.test.js
index e1db63ed..110e5749 100644
--- a/tests/index.test.js
+++ b/tests/index.test.js
@@ -2,6 +2,8 @@ import {
 	getCanonicalLocale,
 	translate,
 	translatePlural,
+	register,
+	_unregister,
 } from '../lib/index'
 
 const setLocale = (locale) => document.documentElement.setAttribute('data-locale', locale)
@@ -78,3 +80,63 @@ describe('getCanonicalLocale', () => {
 		expect(getCanonicalLocale()).toEqual('de-DE')
 	})
 })
+
+describe('register', () => {
+	beforeEach(() => {
+		setLocale('de_DE')
+		window._oc_l10n_registry_translations = undefined
+		window._oc_l10n_registry_plural_functions = undefined
+	})
+
+	it('initial', () => {
+		register('app', {
+			Application: 'Anwendung',
+			'_%n guest_::_%n guests_': ['%n Gast', '%n Gäste'],
+		})
+		expect(translate('app', 'Application')).toBe('Anwendung')
+		expect(translatePlural('app', '%n guest', '%n guests', 1)).toBe('1 Gast')
+		expect(translatePlural('app', '%n guest', '%n guests', 2)).toBe('2 Gäste')
+	})
+
+	it('extend', () => {
+		window._oc_l10n_registry_translations = {
+			app: {
+				Application: 'Anwendung',
+			},
+		}
+		window._oc_l10n_registry_plural_functions = {
+			app: (t) => t === 1 ? 0 : 1,
+		}
+		register('app', {
+			Translation: 'Übersetzung',
+		})
+		expect(translate('app', 'Application')).toBe('Anwendung')
+		expect(translate('app', 'Translation')).toBe('Übersetzung')
+	})
+
+	it('with another app', () => {
+		window._oc_l10n_registry_translations = {
+			core: {
+				'Hello world!': 'Hallo Welt!',
+			},
+		}
+		window._oc_l10n_registry_plural_functions = {
+			core: (t) => t === 1 ? 0 : 1,
+		}
+		register('app', {
+			Application: 'Anwendung',
+		})
+		expect(translate('core', 'Hello world!')).toBe('Hallo Welt!')
+		expect(translate('app', 'Application')).toBe('Anwendung')
+	})
+
+	it('unregister', () => {
+		window._oc_l10n_registry_translations = {}
+		window._oc_l10n_registry_plural_functions = {}
+		register('app', {
+			Application: 'Anwendung',
+		})
+		_unregister('app')
+		expect(translate('app', 'Application')).toBe('Application')
+	})
+})