The Locale and the URL

Since you can store the locale of the user in the session, it may be tempting to use the same URL to display a resource in different languages based on the user's locale. For example, http://www.example.com/contact could show content in English for one user and French for another user. Unfortunately, this violates a fundamental rule of the Web: that a particular URL returns the same resource regardless of the user. To further muddy the problem, which version of the content would be indexed by search engines?

A better policy is to include the locale in the URL using the special _locale parameter:

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# config/routes.yaml
contact:
    path:       /{_locale}/contact
    controller: App\Controller\ContactController::index
    requirements:
        _locale: en|fr|de

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<!-- config/routes.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<routes xmlns="http://symfony.com/schema/routing"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/routing
        https://symfony.com/schema/routing/routing-1.0.xsd">

    <route id="contact" path="/{_locale}/contact">
        controller="App\Controller\ContactController::index">
        <requirement key="_locale">en|fr|de</requirement>
    </route>
</routes>

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// config/routes.php
use App\Controller\ContactController;
use Symfony\Component\Routing\Loader\Configurator\RoutingConfigurator;

return function (RoutingConfigurator $routes) {
    $routes->add('contact', '/{_locale}/contact')
        ->controller([ContactController::class, 'index'])
        ->requirements([
            '_locale' => 'en|fr|de',
        ])
    ;
};

When using the special _locale parameter in a route, the matched locale is automatically set on the Request and can be retrieved via the getLocale() method. In other words, if a user visits the URI /fr/contact, the locale fr will automatically be set as the locale for the current request.

You can now use the locale to create routes to other translated pages in your application.

Setting a Default Locale

What if the user's locale hasn't been determined? You can guarantee that a locale is set on each user's request by defining a default_locale for the framework:

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# config/packages/translation.yaml
framework:
    default_locale: en

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<!-- config/packages/translation.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:framework="http://symfony.com/schema/dic/symfony"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
        https://symfony.com/schema/dic/services/services-1.0.xsd
        http://symfony.com/schema/dic/symfony
        https://symfony.com/schema/dic/symfony/symfony-1.0.xsd">

    <framework:config default-locale="en"/>
</container>

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// config/packages/translation.php
$container->loadFromExtension('framework', [
    'default_locale' => 'en',
]);