use Symfony\Component\Security\Core\Role\SwitchUserRole;
use Symfony\Component\Security\Core\Security;
// ...

public class SomeService
{
    private $security;

    public function __construct(Security $security)
    {
        $this->security = $security;
    }

    public function someMethod()
    {
        // ...

        if ($this->security->isGranted('ROLE_PREVIOUS_ADMIN')) {
            foreach ($this->security->getToken()->getRoles() as $role) {
                if ($role instanceof SwitchUserRole) {
                    $impersonatorUser = $role->getSource()->getUser();
                    break;
                }
            }
        }
    }
}

namespace App\Security\Voter;

use Symfony\Component\Security\Core\Authentication\Token\TokenInterface;
use Symfony\Component\Security\Core\Authorization\Voter\Voter;
use Symfony\Component\Security\Core\User\UserInterface;

class SwitchToCustomerVoter extends Voter
{
    protected function supports($attribute, $subject)
    {
        return in_array($attribute, ['ROLE_ALLOWED_TO_SWITCH'])
            && $subject instanceof UserInterface;
    }

    protected function voteOnAttribute($attribute, $subject, TokenInterface $token)
    {
        $user = $token->getUser();
        // if the user is anonymous or if the subject is not a user, do not grant access
        if (!$user instanceof UserInterface || !$subject instanceof UserInterface) {
            return false;
        }

        if (in_array('ROLE_CUSTOMER', $subject->getRoles())
            && $this->hasSwitchToCustomerRole($token)) {
            return true;
        }

        return false;
    }

    private function hasSwitchToCustomerRole(TokenInterface $token)
    {
        foreach ($token->getRoles() as $role) {
            if ($role->getRole() === 'ROLE_SWITCH_TO_CUSTOMER') {
                return true;
            }
        }

        return false;
    }
}

// src/EventListener/SwitchUserSubscriber.php
namespace App\EventListener;

use Symfony\Component\Security\Http\Event\SwitchUserEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\Security\Http\SecurityEvents;

class SwitchUserSubscriber implements EventSubscriberInterface
{
    public function onSwitchUser(SwitchUserEvent $event)
    {
        $request = $event->getRequest();

        if ($request->hasSession() && ($session = $request->getSession)) {
            $session->set(
                '_locale',
                // assuming your User has some getLocale() method
                $event->getTargetUser()->getLocale()
            );
        }
    }

    public static function getSubscribedEvents()
    {
        return [
            // constant for security.switch_user
            SecurityEvents::SWITCH_USER => 'onSwitchUser',
        ];
    }
}