fix(v2): fix/enhance minor i18n issues reported (#4092)

* fix comment

* allow to pass custom classname in navbar items

* Add IconLanguage comp to dropdown

* do not trim htmlLang

* Add initial hreflang SEO support

* doc hreflang

packages/docusaurus-theme-common/src/index.ts

@@ -16,6 +16,8 @@ export {
   FooterLinkItem,
 } from './utils/useThemeConfig';
 
+export {useAlternatePageUtils} from './utils/useAlternatePageUtils';
+
 export {docVersionSearchTag, DEFAULT_SEARCH_TAG} from './utils/searchUtils';
 
 export {isDocsPluginEnabled} from './utils/docsUtils';

packages/docusaurus-theme-common/src/utils/useAlternatePageUtils.ts

@@ -0,0 +1,50 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import useDocusaurusContext from '@docusaurus/useDocusaurusContext';
+import {useLocation} from '@docusaurus/router';
+
+// Permits to obtain the url of the current page in another locale
+// Useful to generate hreflang meta headers etc...
+// See https://developers.google.com/search/docs/advanced/crawling/localized-versions
+export function useAlternatePageUtils() {
+  const {
+    siteConfig: {baseUrl, url},
+    i18n: {defaultLocale, currentLocale},
+  } = useDocusaurusContext();
+  const {pathname} = useLocation();
+
+  const baseUrlUnlocalized =
+    currentLocale === defaultLocale
+      ? baseUrl
+      : baseUrl.replace(`/${currentLocale}/`, '/');
+
+  const pathnameSuffix = pathname.replace(baseUrl, '');
+
+  function getLocalizedBaseUrl(locale: string) {
+    return locale === defaultLocale
+      ? `${baseUrlUnlocalized}`
+      : `${baseUrlUnlocalized}${locale}/`;
+  }
+
+  // TODO support correct alternate url when localized site is deployed on another domain
+  function createUrl({
+    locale,
+    fullyQualified,
+  }: {
+    locale: string;
+    // For hreflang SEO headers, we need it to be fully qualified (full protocol/domain/path...)
+    // For locale dropdown, using a path is good enough
+    fullyQualified: boolean;
+  }) {
+    return `${fullyQualified ? url : ''}${getLocalizedBaseUrl(
+      locale,
+    )}${pathnameSuffix}`;
+  }
+
+  return {createUrl};
+}