customized version of RouteServiceProvider (Laravel 5.6+) to enable dinamically a prefix (/{lang}/route-name) to support multilanguage, and keeping no prefix for default language.

RouteServiceProvider.php

<?php

namespace App\Providers;

use Illuminate\Support\Facades\Route;
use Illuminate\Foundation\Support\Providers\RouteServiceProvider as ServiceProvider;

class RouteServiceProvider extends ServiceProvider
{
    /**
     * This namespace is applied to your controller routes.
     *
     * In addition, it is set as the URL generator's root namespace.
     *
     * @var string
     */
    protected $namespace = 'App\Http\Controllers';
    protected $prefix = '';

    /**
     * Define your route model bindings, pattern filters, etc.
     *
     * @return void
     */
    public function boot()
    {
        $locale = request()->segment(1);

        if (in_array($locale, config('app.locales'))) {
           app()->setlocale($locale);
           $this->prefix = $locale;
        }
        parent::boot();
    }

    /**
     * Define the routes for the application.
     *
     * @return void
     */
    public function map()
    {
        $this->mapApiRoutes();

        $this->mapWebRoutes();

        //
    }

    /**
     * Define the "web" routes for the application.
     *
     * These routes all receive session state, CSRF protection, etc.
     *
     * @return void
     */
    protected function mapWebRoutes()
    {
            if (!$this->prefix) {

                Route::middleware('web')
                    ->namespace($this->namespace)
                    ->group(base_path('routes/web.php'));
            } else {

                Route::middleware('web')
                    ->prefix($this->prefix)
                    ->namespace($this->namespace)
                    ->group(base_path('routes/web.php'));

            }

    }

    /**
     * Define the "api" routes for the application.
     *
     * These routes are typically stateless.
     *
     * @return void
     */
    protected function mapApiRoutes()
    {
        Route::prefix('api')
             ->middleware('api')
             ->namespace($this->namespace)
             ->group(base_path('routes/api.php'));
    }
}