diff --git a/packages/angular_devkit/build_angular/src/utils/process-bundle.ts b/packages/angular_devkit/build_angular/src/utils/process-bundle.ts
index 3b26aa564541..2640ac083eff 100644
--- a/packages/angular_devkit/build_angular/src/utils/process-bundle.ts
+++ b/packages/angular_devkit/build_angular/src/utils/process-bundle.ts
@@ -634,7 +634,7 @@ export async function inlineLocales(options: InlineOptions) {
       // If locale data is provided, load it and prepend to file
       const localeDataPath = i18n.locales[locale]?.dataPath;
       if (localeDataPath) {
-        localeDataContent = await loadLocaleData(localeDataPath, true);
+        localeDataContent = await loadLocaleData(localeDataPath, true, options.es5);
       }
     }
 
@@ -753,7 +753,7 @@ async function inlineLocalesDirect(ast: ParseResult, options: InlineOptions) {
       let localeDataSource: Source | null = null;
       const localeDataPath = i18n.locales[locale] && i18n.locales[locale].dataPath;
       if (localeDataPath) {
-        const localeDataContent = await loadLocaleData(localeDataPath, true);
+        const localeDataContent = await loadLocaleData(localeDataPath, true, options.es5);
         localeDataSource = new OriginalSource(localeDataContent, path.basename(localeDataPath));
       }
 
@@ -870,19 +870,36 @@ function findLocalizePositions(
   return positions;
 }
 
-async function loadLocaleData(path: string, optimize: boolean): Promise<string> {
+async function loadLocaleData(path: string, optimize: boolean, es5: boolean): Promise<string> {
   // The path is validated during option processing before the build starts
   const content = fs.readFileSync(path, 'utf8');
 
-  // NOTE: This can be removed once the locale data files are preprocessed in the framework
-  if (optimize) {
-    const result = await terserMangle(content, {
-      compress: true,
-      ecma: 5,
-    });
+  // Downlevel and optimize the data
+  const transformResult = await transformAsync(content, {
+    filename: path,
+    // The types do not include the false option even though it is valid
+    // tslint:disable-next-line: no-any
+    inputSourceMap: false as any,
+    babelrc: false,
+    configFile: false,
+    presets: [
+      [
+        require.resolve('@babel/preset-env'),
+        {
+          bugfixes: true,
+          // IE 9 is the oldest supported browser
+          targets: es5 ? { ie: '9' } : { esmodules: true },
+        },
+      ],
+    ],
+    minified: allowMinify && optimize,
+    compact: !shouldBeautify && optimize,
+    comments: !optimize,
+  });
 
-    return result.code;
+  if (!transformResult || !transformResult.code) {
+    throw new Error(`Unknown error occurred processing bundle for "${path}".`);
   }
 
-  return content;
+  return transformResult.code;
 }
diff --git a/tests/legacy-cli/e2e/tests/i18n/ivy-localize-dl-xliff2.ts b/tests/legacy-cli/e2e/tests/i18n/ivy-localize-dl-xliff2.ts
index 28225d45a583..be3ab161c515 100644
--- a/tests/legacy-cli/e2e/tests/i18n/ivy-localize-dl-xliff2.ts
+++ b/tests/legacy-cli/e2e/tests/i18n/ivy-localize-dl-xliff2.ts
@@ -41,6 +41,18 @@ export async function executeTest() {
     await expectFileToMatch(`${outputPath}/vendor-es5.js`, '.ng.common.locales');
     await expectFileToMatch(`${outputPath}/vendor-es2015.js`, '.ng.common.locales');
 
+    // Verify the locale data is browser compatible
+    await expectToFail(() => expectFileToMatch(`${outputPath}/vendor-es5.js`, /\bconst\b/));
+    await expectFileToMatch(`${outputPath}/vendor-es2015.js`, /\bconst\b/);
+
+    // Verify locale data comments are removed in production
+    await expectToFail(() =>
+      expectFileToMatch(`${outputPath}/vendor-es5.js`, '// See angular/tools/gulp-tasks/cldr/extract.js'),
+    );
+    await expectToFail(() =>
+      expectFileToMatch(`${outputPath}/vendor-es2015.js`, '// See angular/tools/gulp-tasks/cldr/extract.js'),
+    );
+
     // Execute Application E2E tests with dev server
     await ng('e2e', `--configuration=${lang}`, '--port=0');