Refactor i18n handling for language cookie and headers

The commit encompasses the aspect of refactoring the i18n handling for language cookies and headers. It also includes the deletion of get-language-cookie file and its transformation into a function inside i18n.server file. Extra functionalities were added to the i18n.server like enhancing the i18n server instance creation to consider the 'accept-language' header and default to environment provided values when necessary. The changes were also adjusted accordingly on the packages/i18n/package.json where deletion of "./cookie" was realized.

apps/web/lib/i18n/i18n.server.ts

@@ -1,10 +1,27 @@
-import getLanguageCookie from '@kit/i18n/cookie';
-import { initializeServerI18n } from '@kit/i18n/server';
+import { cookies, headers } from 'next/headers';
+
+import {
+  getLanguageCookie,
+  initializeServerI18n,
+  parseAcceptLanguageHeader,
+} from '@kit/i18n/server';
 
 import { i18nResolver } from './i18n.resolver';
 
+/**
+ * @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)
+ */
 export function createI18nServerInstance() {
-  const cookie = getLanguageCookie();
+  const acceptLanguage = headers().get('accept-language');
+  const cookie = getLanguageCookie(cookies());
 
-  return initializeServerI18n(cookie, i18nResolver);
+  const language =
+    cookie ?? parseAcceptLanguageHeader(acceptLanguage)[0] ?? undefined;
+
+  return initializeServerI18n(language, i18nResolver);
 }

packages/i18n/package.json

@@ -12,7 +12,6 @@
   "exports": {
     "./server": "./src/i18n.server.ts",
     "./client": "./src/i18n.client.ts",
-    "./cookie": "./src/get-language-cookie.ts",
     "./provider": "./src/i18n-provider.tsx"
   },
   "devDependencies": {

packages/i18n/src/get-language-cookie.ts

@@ -1,9 +0,0 @@
-import { cookies } from 'next/headers';
-
-import { I18N_COOKIE_NAME } from './i18n.settings';
-
-function getLanguageCookie() {
-  return cookies().get(I18N_COOKIE_NAME)?.value;
-}
-
-export default getLanguageCookie;

packages/i18n/src/i18n.server.ts

@@ -2,7 +2,15 @@ import { createInstance } from 'i18next';
 import resourcesToBackend from 'i18next-resources-to-backend';
 import { initReactI18next } from 'react-i18next/initReactI18next';
 
-import { getI18nSettings } from './i18n.settings';
+import { I18N_COOKIE_NAME, getI18nSettings, languages } from './i18n.settings';
+
+export function getLanguageCookie<
+  Cookies extends {
+    get: (name: string) => { value: string } | undefined;
+  },
+>(cookies: Cookies) {
+  return cookies.get(I18N_COOKIE_NAME)?.value;
+}
 
 export async function initializeServerI18n(
   lang: string | undefined,
@@ -33,3 +41,39 @@ export async function initializeServerI18n(
 
   return i18nInstance;
 }
+
+export function parseAcceptLanguageHeader(
+  languageHeaderValue: string | null | undefined,
+  acceptedLanguages = languages,
+): string[] {
+  // Return an empty array if the header value is not provided
+  if (!languageHeaderValue) return [];
+
+  const ignoreWildcard = true;
+
+  // Split the header value by comma and map each language to its quality value
+  return languageHeaderValue
+    .split(',')
+    .map((lang): [number, string] => {
+      const [locale, q = 'q=1'] = lang.split(';');
+
+      if (!locale) return [0, ''];
+
+      const trimmedLocale = locale.trim();
+      const numQ = Number(q.replace(/q ?=/, ''));
+
+      return [isNaN(numQ) ? 0 : numQ, trimmedLocale];
+    })
+    .sort(([q1], [q2]) => q2 - q1) // Sort by quality value in descending order
+    .flatMap(([_, locale]) => {
+      // Ignore wildcard '*' if 'ignoreWildcard' is true
+      if (locale === '*' && ignoreWildcard) return [];
+
+      // Return the locale if it's included in the accepted languages
+      try {
+        return acceptedLanguages.includes(locale) ? [locale] : [];
+      } catch {
+        return [];
+      }
+    });
+}

packages/i18n/src/i18n.settings.ts

@@ -1,7 +1,7 @@
 import { InitOptions } from 'i18next';
 
 const fallbackLng = 'en';
-const languages: string[] = [fallbackLng];
+export const languages: string[] = [fallbackLng];
 
 export const I18N_COOKIE_NAME = 'lang';