Queued jobs in different locales. See https://github.com/laravel/internals/issues/394 for more info.

CombinedNotification.php

<?php

namespace App\Notifications;

use App\Helpers\RemembersLocale;
use Illuminate\Bus\Queueable;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Notifications\Messages\MailMessage;
use Illuminate\Notifications\Notification;
use Notifications\Channels\FirebaseChannel;
use Notifications\Messages\FirebaseMessage;
use Notifications\Notifications\FirebaseNotification;

class CombinedNotification extends Notification implements FirebaseNotification, ShouldQueue
{
    use Queueable;
    use RemembersLocale;

    public function __construct()
    {
        // When the job gets created and dispatched, we store
        // the current application's locale in this job. This
        // way, we can reload it later when it's being executed
        // in a queue for instance.
        $this->rememberLocale();
    }

    /**
     * Get the notification's delivery channels.
     *
     * @param mixed $notifiable
     *
     * @return array
     */
    public function via($notifiable) : array
    {
        return [
            'mail',
            FirebaseChannel::class,
        ];
    }

    /**
     * @param $notifiable
     *
     * @return \Notifications\Messages\FirebaseMessage
     */
    public function toFcm($notifiable) : FirebaseMessage
    {
        // A Firebase (push) notification is a primary example of how to use
        // the RemembersLocale trait. We wrap our notification in a closure
        // to have all our copy translated in the locale we saved earlier.

        return $this->executeInLocale(function () use ($notifiable) {
            return app(FirebaseMessage::class)
                ->setTitle(__('combined_notification.title'))
                ->setBody(__('combined_notification.body'));
        });
    }

    /**
     * Get the mail representation of the notification.
     *
     * @param $notifiable
     *
     * @return \Illuminate\Notifications\Messages\MailMessage
     */
    public function toMail($notifiable)
    {
        // An e-mail notification or mailable is different in the way its built.
        // We need to set the locale for the entire duration of the process so 
        // the framework parses the returned mail message in that locale. If we
        // just wrap it in the closure like any other notification, it won't 
        // get translated because we'd have reset the locale before the message
        // is built (which is some time after it gets returned here).

        $this->restoreLocale();

        return (new MailMessage)
            ->subject(__('combined_notification.subject'))
            ->markdown('emails.combined_notification');
    }
}

RemembersLocale.php

  <?php

namespace App\Helpers;

trait RemembersLocale
{
    /**
     * The queued job's locale (language).
     *
     * @var string
     */
    public $locale;

    /**
     * Manually set the locale.
     *
     * @param string $locale
     */
    public function setLocale(string $locale)
    {
        $this->locale = $locale;
    }

    /**
     * Remember the current application's locale.
     */
    protected function rememberLocale()
    {
        $this->locale = app()->getLocale();
    }

    /**
     * Restore the saved locale.
     */
    protected function restoreLocale()
    {
        app()->setLocale($this->locale);
    }

    /**
     * @param callable $callback
     *
     * @return mixed
     */
    protected function executeInLocale(callable $callback)
    {
        $locale = app()->getLocale();

        $this->restoreLocale();

        $result = $callback();

        app()->setLocale($locale);

        return $result;
    }
}