Add locale detection guide

docs/locale_detection.md

@@ -0,0 +1,88 @@
+---
+id: locale_detection
+title: Locale detection
+sidebar_label: Locale detection
+---
+
+# How to detect the user's locale from the browser?
+
+## Installation
+To implement locale detection in your PHP project, you can use the `HTTP2` library. You can install it via Composer:
+
+```shell
+composer require pear/http2
+```
+
+# Usage
+After installing the `HTTP2 library`, you can utilize it in your PHP code to detect the user's locale. Here's a basic example:
+
+```php
+<?php
+
+namespace App\Http\Middleware;
+
+use Closure;
+use fbt\FbtConfig;
+use fbt\Runtime\Shared\FbtHooks;
+use Illuminate\Http\Request;
+
+class RegisterFbt
+{
+    /**
+     * Handle an incoming request.
+     *
+     * @param  \Illuminate\Http\Request  $request
+     * @param  \Closure  $next
+     * @return mixed
+     */
+    public function handle(Request $request, Closure $next)
+    {
+        if ($request->user()) {
+            // Notice: locale of logged-in user is always preferred
+            FbtConfig::set('viewerContext', $request->user());
+        } else {
+            $supportedLanguages = [
+                'en'    => 'en_US',
+                'en-UK' => 'en_UK',
+                'de'    => 'de_DE',
+                'de-AT' => 'de_AT',
+                'cs'    => 'cs_CZ',
+                'sk'    => 'sk_SK',
+            ];
+
+            $http = new \HTTP2();
+
+            // Detect the user's language
+            $clientLanguage = $http->negotiateLanguage($supportedLanguages, $fallbackLanguage = 'en');
+
+            // Get the locale for the detected language
+            $locale = $supportedLanguages[$clientLanguage];
+
+            // Set the detected locale
+            FbtConfig::set('locale', $locale);
+        }
+
+        return $next($request);
+    }
+}
+```
+
+then register the middleware in `app/Http/Kernel.php`:
+
+```php
+protected $middlewareGroups = [
+    'web' => [
+        // ...
+        \App\Http\Middleware\RegisterFbt::class,
+    ],
+    // ...
+];
+```
+
+# Explanation
+
+## HTTP Accept-Language Header
+The `negotiateLanguage()` method works by parsing the HTTP `Accept-Language` header sent by the user's browser. This header contains information about the user's preferred languages in order of priority. The method then matches these languages against the provided list and returns the best match.
+
+## Fallback Locale
+In case the user's preferred language is not available or cannot be determined, it's essential to have a fallback locale. This ensures that your application always defaults to a suitable language/locale even when the detection process fails.