[9.x] Use get methods to access application locale

[abenevaut]

This PR allows to easily mock access to config (app.locale & app.fallback_locale) in TranslationServiceProvider.

I got this error

Error: Cannot use object of type Mockery_0_stdClass as array

/vendor/laravel/framework/src/Illuminate/Translation/TranslationServiceProvider.php:25

When I apply similar mock as https://github.com/laravel/framework/blob/9.x/tests/Foundation/FoundationApplicationTest.php#L22

class CommandTest extends TestCase
{
    protected Application $app;

    protected function setUp(): void
    {
        $this->app = new Application();
        $this->app['config'] = m::mock(\stdClass::class);
        $this->app['env'] = 'testing';

        $this->app->register(ValidationServiceProvider::class);
        $this->app->register(TranslationServiceProvider::class);
        $this->app->register(FilesystemServiceProvider::class);

        $this->app->instance('path.config', __DIR__ . '/../config');
        $this->app->instance('path.storage', __DIR__ . '/../storage');
        $this->app->instance('path.lang', __DIR__ . '/../lang');

        $this->app->boot();

        $this
            ->app['config']
            ->shouldReceive('get')
            ->once()
            ->with('app.locale')
            ->andReturns('en');
        $this
            ->app['config']
            ->shouldReceive('get')
            ->once()
            ->with('app.fallback_locale')
            ->andReturns('en');

        Facade::setFacadeApplication($this->app);
    }