B2B-88: add starter kit structure and elements

lib/i18n/i18n.server.ts

@@ -0,0 +1,98 @@
+import 'server-only';
+
+import { cache } from 'react';
+
+import { cookies, headers } from 'next/headers';
+
+import { z } from 'zod';
+
+import {
+  initializeServerI18n,
+  parseAcceptLanguageHeader,
+} from '@kit/i18n/server';
+
+import featuresFlagConfig from '~/config/feature-flags.config';
+import {
+  I18N_COOKIE_NAME,
+  getI18nSettings,
+  languages,
+} from '~/lib/i18n/i18n.settings';
+
+import { i18nResolver } from './i18n.resolver';
+
+/**
+ * @name priority
+ * @description The language priority setting from the feature flag configuration.
+ */
+const priority = featuresFlagConfig.languagePriority;
+
+/**
+ * @name createI18nServerInstance
+ * @description Creates an instance of the i18n server.
+ * It uses the language from the cookie if it exists, otherwise it uses the language from the accept-language header.
+ * If neither is available, it will default to the provided environment variable.
+ *
+ * Initialize the i18n instance for every RSC server request (eg. each page/layout)
+ */
+async function createInstance() {
+  const cookieStore = await cookies();
+  const langCookieValue = cookieStore.get(I18N_COOKIE_NAME)?.value;
+
+  let selectedLanguage: string | undefined = undefined;
+
+  // if the cookie is set, use the language from the cookie
+  if (langCookieValue) {
+    selectedLanguage = getLanguageOrFallback(langCookieValue);
+  }
+
+  // if not, check if the language priority is set to user and
+  // use the user's preferred language
+  if (!selectedLanguage && priority === 'user') {
+    const userPreferredLanguage = await getPreferredLanguageFromBrowser();
+
+    selectedLanguage = getLanguageOrFallback(userPreferredLanguage);
+  }
+
+  const settings = getI18nSettings(selectedLanguage);
+
+  return initializeServerI18n(settings, i18nResolver);
+}
+
+export const createI18nServerInstance = cache(createInstance);
+
+/**
+ * @name getPreferredLanguageFromBrowser
+ * Get the user's preferred language from the accept-language header.
+ */
+async function getPreferredLanguageFromBrowser() {
+  const headersStore = await headers();
+  const acceptLanguage = headersStore.get('accept-language');
+
+  // no accept-language header, return
+  if (!acceptLanguage) {
+    return;
+  }
+
+  return parseAcceptLanguageHeader(acceptLanguage, languages)[0];
+}
+
+/**
+ * @name getLanguageOrFallback
+ * Get the language or fallback to the default language.
+ * @param selectedLanguage
+ */
+function getLanguageOrFallback(selectedLanguage: string | undefined) {
+  const language = z
+    .enum(languages as [string, ...string[]])
+    .safeParse(selectedLanguage);
+
+  if (language.success) {
+    return language.data;
+  }
+
+  console.warn(
+    `The language passed is invalid. Defaulted back to "${languages[0]}"`,
+  );
+
+  return languages[0];
+}