THIS ANSWER IS ONLY VALID FOR LARAVEL <=5.1 OR WHEN YOU WANT TO USE MANY DIFFERENT LOCALES see this answer for a solution in Laravel >=5.2.

To use a locale with Faker, the generator needs creating with the locale.

$faker = Faker\Factory::create('fr_FR'); // create a French faker

From the faker documentation:

If no localized provider is found, the factory fallbacks to the default locale (en_EN).

Laravel by default, binds the creation of a faker instance in the EloquentServiceProvider. The exact code used to bind is:

// FakerFactory is aliased to Faker\Factory
$this->app->singleton(FakerGenerator::class, function () {
    return FakerFactory::create();
});

It would appear that Laravel has no way to modify the locale of the faker instance it passes into the factory closures as it does not pass in any arguments to Faker.

As such you would be better served by using your own instance of Faker in the factory method.

$localisedFaker = Faker\Factory::create("fr_FR");

$factory->define(App\Flyer::class, function (Faker\Generator $faker) {

    // Now use the localisedFaker instead of the Faker\Generator
    // passed in to the closure.
    return [
        'zip' => $localisedFaker->postcode,
        'state' => $localisedFaker->state,  
    ];
});

Try this way

$factory->define(App\Post::class, function () {
    $faker = Faker\Factory::create('fa_IR');

    return [
        'post_title'        => $faker->sentence,
        'post_content'      => $faker->paragraph,
        'post_content_full' => $faker->paragraph(3),
        'author'            => $faker->name
    ];
});