[REFACTOR] 實作側邊欄與儀表板多語系化，修復 UI 位移與樣式優化

app/Http/Controllers/System/LanguageController.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace App\Http\Controllers\System;
+
+use App\Http\Controllers\Controller;
+use Illuminate\Http\Request;
+use Illuminate\Support\Facades\Session;
+
+class LanguageController extends Controller
+{
+    /**
+     * Switch application language.
+     *
+     * @param string $locale
+     * @return \Illuminate\Http\RedirectResponse
+     */
+    public function switch($locale)
+    {
+        if (in_array($locale, ['en', 'zh_TW', 'ja'])) {
+            Session::put('locale', $locale);
+        }
+
+        return redirect()->back();
+    }
+}