diff --git a/packages/ckeditor5-dev-utils/lib/translations/multiplelanguagetranslationservice.js b/packages/ckeditor5-dev-utils/lib/translations/multiplelanguagetranslationservice.js
index bc5e7d07b..6f02d390d 100644
--- a/packages/ckeditor5-dev-utils/lib/translations/multiplelanguagetranslationservice.js
+++ b/packages/ckeditor5-dev-utils/lib/translations/multiplelanguagetranslationservice.js
@@ -176,13 +176,13 @@ module.exports = class MultipleLanguageTranslationService extends EventEmitter {
 		}
 
 		const mainAssetName = compilationAssetNames[ 0 ];
-		const mainCompilationAsset = compilationAssets[ mainAssetName ];
 
 		const mainTranslationAsset = this._getTranslationAssets( outputDirectory, [ this._mainLanguage ] )[ 0 ];
 
 		const mergedCompilationAsset = {
-			outputBody: mainCompilationAsset.source() + '\n;' + mainTranslationAsset.outputBody,
-			outputPath: mainAssetName
+			outputBody: mainTranslationAsset.outputBody,
+			outputPath: mainAssetName,
+			shouldConcat: true
 		};
 
 		const otherLanguages = Array.from( this._languages )
@@ -211,7 +211,16 @@ module.exports = class MultipleLanguageTranslationService extends EventEmitter {
 			const stringifiedTranslations = JSON.stringify( translatedStrings )
 				.replace( /"([a-z]+)":/g, '$1:' );
 
-			const outputBody = `CKEDITOR_TRANSLATIONS.add('${ language }',${ stringifiedTranslations })`;
+			const outputBody = (
+				// We need to ensure that the CKEDITOR_TRANSLATIONS variable exists and if it exists, we need to extend it.
+				// Use ES5 because this bit will not be transpiled!
+				'(function(d){' +
+					`d['${ language }']=Object.assign(` +
+						`d['${ language }']||{},` +
+						`${ stringifiedTranslations }` +
+					')' +
+				'})(window.CKEDITOR_TRANSLATIONS||(window.CKEDITOR_TRANSLATIONS={}));'
+			);
 
 			return { outputBody, outputPath };
 		} );
diff --git a/packages/ckeditor5-dev-utils/tests/translations/multiplelanguagetranslationservice.js b/packages/ckeditor5-dev-utils/tests/translations/multiplelanguagetranslationservice.js
index 22ea81907..cbf5b9935 100644
--- a/packages/ckeditor5-dev-utils/tests/translations/multiplelanguagetranslationservice.js
+++ b/packages/ckeditor5-dev-utils/tests/translations/multiplelanguagetranslationservice.js
@@ -217,11 +217,14 @@ describe( 'translations', () => {
 				expect( assets ).to.deep.equal( [
 					{
 						outputPath: 'ckeditor.js',
-						outputBody: 'source\n;CKEDITOR_TRANSLATIONS.add(\'pl\',{a:"Anuluj",b:"Zapisz"})'
+						outputBody: '(function(d){d[\'pl\']=Object.assign(d[\'pl\']||{},{a:"Anuluj",b:"Zapisz"})})' +
+							'(window.CKEDITOR_TRANSLATIONS||(window.CKEDITOR_TRANSLATIONS={}));',
+						shouldConcat: true
 					},
 					{
 						outputPath: path.join( 'lang', 'en.js' ),
-						outputBody: 'CKEDITOR_TRANSLATIONS.add(\'en\',{a:"Cancel",b:"Save"})'
+						outputBody: '(function(d){d[\'en\']=Object.assign(d[\'en\']||{},{a:"Cancel",b:"Save"})})' +
+							'(window.CKEDITOR_TRANSLATIONS||(window.CKEDITOR_TRANSLATIONS={}));'
 					}
 				] );
 			} );
@@ -283,11 +286,14 @@ describe( 'translations', () => {
 				expect( assets ).to.deep.equal( [
 					{
 						outputPath: 'ckeditor.js',
-						outputBody: 'source\n;CKEDITOR_TRANSLATIONS.add(\'pl\',{a:"Anuluj",b:"Zapisz"})'
+						outputBody: '(function(d){d[\'pl\']=Object.assign(d[\'pl\']||{},{a:"Anuluj",b:"Zapisz"})})' +
+							'(window.CKEDITOR_TRANSLATIONS||(window.CKEDITOR_TRANSLATIONS={}));',
+						shouldConcat: true
 					},
 					{
 						outputPath: path.join( 'lang', 'xxx.js' ),
-						outputBody: 'CKEDITOR_TRANSLATIONS.add(\'xxx\',{a:"Cancel",b:"Save"})'
+						outputBody: '(function(d){d[\'xxx\']=Object.assign(d[\'xxx\']||{},{a:"Cancel",b:"Save"})})' +
+							'(window.CKEDITOR_TRANSLATIONS||(window.CKEDITOR_TRANSLATIONS={}));'
 					}
 				] );
 			} );
@@ -381,7 +387,9 @@ describe( 'translations', () => {
 				expect( assets ).to.deep.equal( [
 					{
 						outputPath: 'ckeditor.js',
-						outputBody: 'source\n;CKEDITOR_TRANSLATIONS.add(\'pl\',{a:"Anuluj",b:"Zapisz"})'
+						outputBody: '(function(d){d[\'pl\']=Object.assign(d[\'pl\']||{},{a:"Anuluj",b:"Zapisz"})})' +
+							'(window.CKEDITOR_TRANSLATIONS||(window.CKEDITOR_TRANSLATIONS={}));',
+						shouldConcat: true
 					}
 				] );
 			} );
@@ -421,7 +429,8 @@ describe( 'translations', () => {
 				expect( assets ).to.deep.equal( [
 					{
 						outputPath: path.join( 'lang', 'pl.js' ),
-						outputBody: 'CKEDITOR_TRANSLATIONS.add(\'pl\',{a:"Anuluj",b:"Zapisz"})'
+						outputBody: '(function(d){d[\'pl\']=Object.assign(d[\'pl\']||{},{a:"Anuluj",b:"Zapisz"})})' +
+							'(window.CKEDITOR_TRANSLATIONS||(window.CKEDITOR_TRANSLATIONS={}));'
 					}
 				] );
 			} );
@@ -457,11 +466,14 @@ describe( 'translations', () => {
 				expect( assets ).to.deep.equal( [
 					{
 						outputPath: 'ckeditor.js',
-						outputBody: 'source\n;CKEDITOR_TRANSLATIONS.add(\'pl\',{a:"Anuluj"})'
+						outputBody: '(function(d){d[\'pl\']=Object.assign(d[\'pl\']||{},{a:"Anuluj"})})' +
+							'(window.CKEDITOR_TRANSLATIONS||(window.CKEDITOR_TRANSLATIONS={}));',
+						shouldConcat: true
 					},
 					{
 						outputPath: path.join( 'custom-lang-path', 'en.js' ),
-						outputBody: 'CKEDITOR_TRANSLATIONS.add(\'en\',{a:"Cancel"})'
+						outputBody: '(function(d){d[\'en\']=Object.assign(d[\'en\']||{},{a:"Cancel"})})' +
+							'(window.CKEDITOR_TRANSLATIONS||(window.CKEDITOR_TRANSLATIONS={}));'
 					}
 				] );
 			} );
@@ -538,11 +550,14 @@ describe( 'translations', () => {
 				expect( assets ).to.deep.equal( [
 					{
 						outputPath: 'ckeditor.js',
-						outputBody: 'source\n;CKEDITOR_TRANSLATIONS.add(\'pl\',{a:"Zapisz"})'
+						outputBody: '(function(d){d[\'pl\']=Object.assign(d[\'pl\']||{},{a:"Zapisz"})})' +
+							'(window.CKEDITOR_TRANSLATIONS||(window.CKEDITOR_TRANSLATIONS={}));',
+						shouldConcat: true
 					},
 					{
 						outputPath: path.join( 'lang', 'de.js' ),
-						outputBody: 'CKEDITOR_TRANSLATIONS.add(\'de\',{a:"Speichern"})'
+						outputBody: '(function(d){d[\'de\']=Object.assign(d[\'de\']||{},{a:"Speichern"})})' +
+							'(window.CKEDITOR_TRANSLATIONS||(window.CKEDITOR_TRANSLATIONS={}));'
 					}
 				] );
 			} );
diff --git a/packages/ckeditor5-dev-webpack-plugin/lib/servetranslations.js b/packages/ckeditor5-dev-webpack-plugin/lib/servetranslations.js
index cb5741150..5c6611820 100644
--- a/packages/ckeditor5-dev-webpack-plugin/lib/servetranslations.js
+++ b/packages/ckeditor5-dev-webpack-plugin/lib/servetranslations.js
@@ -9,6 +9,7 @@ const chalk = require( 'chalk' );
 const rimraf = require( 'rimraf' );
 const fs = require( 'fs' );
 const path = require( 'path' );
+const { RawSource, ConcatSource } = require( 'webpack-sources' );
 
 /**
  * Serve translations depending on the used translation service and passed options.
@@ -77,20 +78,37 @@ module.exports = function serveTranslations( compiler, options, translationServi
 	} );
 
 	// At the end of the compilation add assets generated from the PO files.
-	compiler.plugin( 'emit', ( compilation, done ) => {
-		const generatedAssets = translationService.getAssets( {
-			outputDirectory: options.outputDirectory,
-			compilationAssets: compilation.assets
-		} );
-
-		for ( const asset of generatedAssets ) {
-			compilation.assets[ asset.outputPath ] = {
-				source: () => asset.outputBody,
-				size: () => asset.outputBody.length,
-			};
-		}
+	// Use `optimize-chunk-assets` instead of `emit` to emit assets before the `webpack.BannerPlugin`.
+	compiler.plugin( 'compilation', compilation => {
+		compilation.plugin( 'optimize-chunk-assets', ( chunks, done ) => {
+			const generatedAssets = translationService.getAssets( {
+				outputDirectory: options.outputDirectory,
+				compilationAssets: compilation.assets
+			} );
+
+			const allFiles = chunks.reduce( ( acc, chunk ) => [ ...acc, ...chunk.files ], [] );
+
+			for ( const asset of generatedAssets ) {
+				if ( asset.shouldConcat ) {
+					// We need to concat sources here to support source maps for CKE5 code.
+					const originalAsset = compilation.assets[ asset.outputPath ];
+					compilation.assets[ asset.outputPath ] = new ConcatSource( asset.outputBody, '\n', originalAsset );
+				} else {
+					const chunkExists = allFiles.includes( asset.outputPath );
+
+					if ( !chunkExists ) {
+						// RawSource is used when corresponding chunk does not exist.
+						compilation.assets[ asset.outputPath ] = new RawSource( asset.outputBody );
+					} else {
+						// String is used when corresponding chunk exists and maintain proper sourcemaps.
+						// Changing to RawSource would drop source maps.
+						compilation.assets[ asset.outputPath ] = asset.outputBody;
+					}
+				}
+			}
 
-		done();
+			done();
+		} );
 	} );
 
 	function emitError( error ) {