diff --git a/packages/ckeditor5-utils/src/dom/global.ts b/packages/ckeditor5-utils/src/dom/global.ts
index 55147ec4ba8..20f0dd849b5 100644
--- a/packages/ckeditor5-utils/src/dom/global.ts
+++ b/packages/ckeditor5-utils/src/dom/global.ts
@@ -23,4 +23,21 @@
  *
  *		console.log( global.window.innerWidth );
  */
-export default { window, document };
+
+let global: { window: Window & typeof globalThis; document: Document };
+
+// In some environments window and document API might not be available.
+try {
+	global = { window, document };
+} catch ( e ) {
+	// It's not possible to mock a window object to simulate lack of a window object without writing extremely convoluted code.
+	/* istanbul ignore next */
+
+	// Let's cast it to not change module's API.
+	// We only handle this so loading editor in environments without window and document doesn't fail.
+	// For better DX we shouldn't introduce mixed types and require developers to check the type manually.
+	// This module should not be used on purpose in any environment outside browser.
+	global = { window: {} as any, document: {} as any };
+}
+
+export default global;
diff --git a/packages/ckeditor5-utils/src/env.ts b/packages/ckeditor5-utils/src/env.ts
index 0c4d0951f7e..afa49262ba7 100644
--- a/packages/ckeditor5-utils/src/env.ts
+++ b/packages/ckeditor5-utils/src/env.ts
@@ -9,7 +9,22 @@
  * @module utils/env
  */
 
-const userAgent = navigator.userAgent.toLowerCase();
+/**
+ * Safely returns `userAgent` from browser's navigator API in a lower case.
+ * If navigator API is not available it will return an empty string.
+ *
+ * @returns {String}
+ */
+export function getUserAgent( ): string {
+	// In some environments navigator API might not be available.
+	try {
+		return navigator.userAgent.toLowerCase();
+	} catch ( e ) {
+		return '';
+	}
+}
+
+const userAgent = getUserAgent();
 
 /**
  * A namespace containing environment and browser information.
diff --git a/packages/ckeditor5-utils/src/translation-service.ts b/packages/ckeditor5-utils/src/translation-service.ts
index b8d93ef1d01..4e08971e316 100644
--- a/packages/ckeditor5-utils/src/translation-service.ts
+++ b/packages/ckeditor5-utils/src/translation-service.ts
@@ -3,7 +3,6 @@
  * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-oss-license
  */
 
-/* globals window */
 /* eslint-disable no-var */
 
 /**
@@ -11,6 +10,7 @@
  */
 
 import CKEditorError from './ckeditorerror';
+import global from './dom/global';
 
 declare global {
 	var CKEDITOR_TRANSLATIONS: {
@@ -22,8 +22,8 @@ declare global {
 }
 
 /* istanbul ignore else */
-if ( !window.CKEDITOR_TRANSLATIONS ) {
-	window.CKEDITOR_TRANSLATIONS = {};
+if ( !global.window.CKEDITOR_TRANSLATIONS ) {
+	global.window.CKEDITOR_TRANSLATIONS = {};
 }
 
 /**
@@ -78,15 +78,15 @@ if ( !window.CKEDITOR_TRANSLATIONS ) {
  * the one below:
  *
  *		function addTranslations( language, translations, getPluralForm ) {
- *			if ( !window.CKEDITOR_TRANSLATIONS ) {
- *				window.CKEDITOR_TRANSLATIONS = {};
+ *			if ( !global.window.CKEDITOR_TRANSLATIONS ) {
+ *				global.window.CKEDITOR_TRANSLATIONS = {};
  *			}
 
- *			if ( !window.CKEDITOR_TRANSLATIONS[ language ] ) {
- *				window.CKEDITOR_TRANSLATIONS[ language ] = {};
+ *			if ( !global.window.CKEDITOR_TRANSLATIONS[ language ] ) {
+ *				global.window.CKEDITOR_TRANSLATIONS[ language ] = {};
  *			}
  *
- *			const languageTranslations = window.CKEDITOR_TRANSLATIONS[ language ];
+ *			const languageTranslations = global.window.CKEDITOR_TRANSLATIONS[ language ];
  *
  * 			languageTranslations.dictionary = languageTranslations.dictionary || {};
  * 			languageTranslations.getPluralForm = getPluralForm || languageTranslations.getPluralForm;
@@ -106,11 +106,11 @@ export function add(
 	translations: { readonly [ messageId: string ]: string | readonly string[] },
 	getPluralForm?: ( n: number ) => number
 ): void {
-	if ( !window.CKEDITOR_TRANSLATIONS[ language ] ) {
-		window.CKEDITOR_TRANSLATIONS[ language ] = {} as any;
+	if ( !global.window.CKEDITOR_TRANSLATIONS[ language ] ) {
+		global.window.CKEDITOR_TRANSLATIONS[ language ] = {} as any;
 	}
 
-	const languageTranslations = window.CKEDITOR_TRANSLATIONS[ language ];
+	const languageTranslations = global.window.CKEDITOR_TRANSLATIONS[ language ];
 
 	languageTranslations.dictionary = languageTranslations.dictionary || {};
 	languageTranslations.getPluralForm = getPluralForm || languageTranslations.getPluralForm;
@@ -166,7 +166,7 @@ export function _translate( language: string, message: Message, quantity: number
 	if ( numberOfLanguages === 1 ) {
 		// Override the language to the only supported one.
 		// This can't be done in the `Locale` class, because the translations comes after the `Locale` class initialization.
-		language = Object.keys( window.CKEDITOR_TRANSLATIONS )[ 0 ];
+		language = Object.keys( global.window.CKEDITOR_TRANSLATIONS )[ 0 ];
 	}
 
 	const messageId = message.id || message.string;
@@ -180,8 +180,8 @@ export function _translate( language: string, message: Message, quantity: number
 		return message.string;
 	}
 
-	const dictionary = window.CKEDITOR_TRANSLATIONS[ language ].dictionary;
-	const getPluralForm = window.CKEDITOR_TRANSLATIONS[ language ].getPluralForm || ( n => n === 1 ? 0 : 1 );
+	const dictionary = global.window.CKEDITOR_TRANSLATIONS[ language ].dictionary;
+	const getPluralForm = global.window.CKEDITOR_TRANSLATIONS[ language ].getPluralForm || ( n => n === 1 ? 0 : 1 );
 	const translation = dictionary[ messageId ];
 
 	if ( typeof translation === 'string' ) {
@@ -200,19 +200,19 @@ export function _translate( language: string, message: Message, quantity: number
  * @protected
  */
 export function _clear(): void {
-	window.CKEDITOR_TRANSLATIONS = {};
+	global.window.CKEDITOR_TRANSLATIONS = {};
 }
 
 // Checks whether the dictionary exists and translation in that dictionary exists.
 function hasTranslation( language: string, messageId: string ): boolean {
 	return (
-		!!window.CKEDITOR_TRANSLATIONS[ language ] &&
-		!!window.CKEDITOR_TRANSLATIONS[ language ].dictionary[ messageId ]
+		!!global.window.CKEDITOR_TRANSLATIONS[ language ] &&
+		!!global.window.CKEDITOR_TRANSLATIONS[ language ].dictionary[ messageId ]
 	);
 }
 
 function getNumberOfLanguages(): number {
-	return Object.keys( window.CKEDITOR_TRANSLATIONS ).length;
+	return Object.keys( global.window.CKEDITOR_TRANSLATIONS ).length;
 }
 
 /**
diff --git a/packages/ckeditor5-utils/tests/env.js b/packages/ckeditor5-utils/tests/env.js
index eb6c83cd0e1..0d4c9271a0f 100644
--- a/packages/ckeditor5-utils/tests/env.js
+++ b/packages/ckeditor5-utils/tests/env.js
@@ -4,7 +4,7 @@
  */
 
 import env, {
-	isMac, isWindows, isGecko, isSafari, isiOS, isAndroid, isRegExpUnicodePropertySupported, isBlink
+	isMac, isWindows, isGecko, isSafari, isiOS, isAndroid, isRegExpUnicodePropertySupported, isBlink, getUserAgent
 } from '../src/env';
 
 import global from '../src/dom/global';
@@ -286,4 +286,26 @@ describe( 'Env', () => {
 			}
 		} );
 	} );
+
+	describe( 'getUserAgent()', () => {
+		it( 'should return user agent in lower case', () => {
+			sinon.stub( global.window.navigator, 'userAgent' ).value( 'CKBrowser' );
+
+			expect( getUserAgent() ).to.equal( 'ckbrowser' );
+		} );
+
+		it( 'should return empty string if navigator API is unavailable', () => {
+			sinon.stub( global.window, 'navigator' ).value( undefined );
+
+			expect( getUserAgent() ).to.equal( '' );
+		} );
+
+		it( 'should not throw an error if navigator API is unavailable', () => {
+			sinon.stub( global.window, 'navigator' ).value( undefined );
+
+			expect( () => {
+				getUserAgent();
+			} ).to.not.throw();
+		} );
+	} );
 } );