你传递的参数是什么。我看到的都是没有参数的route-url
如果你愿意，你可以 checkout 这个URL，或者直接从Laravel文档中检查这个块
有时候，您可能需要指定一个路由参数，而该参数可能并不总是出现在URI中。您可以通过放置？参数名称后的标记。确保给予路由的相应变量指定默认值：

Route::get('/user/{name?}', function ($name = null) {
    return $name;
});

Route::get('/user/{name?}', function ($name = 'John') {
    return $name;
});

我假设你也在使用最新版本的Laravel，这里是使用参数Go To Docs进行路由的文档链接。
这里是确切的标签，你可以找到我的片段Laravel参数

在你的情况下有一个小误会。
您的路由不接受任何参数，您正在处理请求。在这种情况下，解决方案应该是下一个：

$language = $request->query('lang');
    if (null !== $language) {
        App::setLocale($language);
    }

    return $next($request);

终于解决了这个问题，现在你可以在中间件中获取lang参数了
web.php

<?php

use Illuminate\Support\Facades\Route;
use App\Http\Middleware\MyMiddleware;

/*
|--------------------------------------------------------------------------
| Web Routes
|--------------------------------------------------------------------------
|
| Here is where you can register web routes for your application. These
| routes are loaded by the RouteServiceProvider within a group which
| contains the "web" middleware group. Now create something great!
|
*/


Route::middleware([MyMiddleware::class])->group(function () {
    Route::get('lang/change', function () {
        return view('welcome');
    })->name('changeLang');
});

welcome.blade.php

<div class="col-md-2 col-md-offset-6 text-right">
    <strong>Select Language: </strong>
</div>
<div class="col-md-4">
    <select class="form-control changeLang">
        <option value="en" {{ $_GET['lang'] == 'en' ? 'selected' : '' }}>English</option>
        <option value="ar" {{ $_GET['lang'] == 'ar' ? 'selected' : '' }}>Arabic</option>
    </select>
</div>
{{-- @endauth --}}
</ul>
</div>

{{-- <span class="text-white">{{ auth()->user() !=null ? auth()->user()->name : "Guest" }}</span> --}}

</div>
</nav>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js" type="text/javascript"></script>

<script type="text/javascript">

var url = "{{ route('changeLang') }}";

$(".changeLang").change(function(){
window.location.href = url + "?lang="+ $(this).val();
});

</script>

MyMiddleware.php

<?php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Http\Request;

class MyMiddleware
{
    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure(\Illuminate\Http\Request): (\Illuminate\Http\Response|\Illuminate\Http\RedirectResponse)  $next
     * @return \Illuminate\Http\Response|\Illuminate\Http\RedirectResponse
     */
    public function handle(Request $request, Closure $next)
    {
        // you can use 
        print_r($_GET);

        // or you can use this also 
        print_r($request->query('lang'));

        // or you can use this also 
        print_r($request->all());
        
        return $next($request);
    }
}