PSR-7 Localization POC. A symfony translator is used but the zend-i18n translator has a similar setup.

App\Core\I18n\I18nUrlHelper.php

<?php

namespace App\Core\I18n;

use Zend\Expressive\Helper\UrlHelper;

class I18nUrlHelper extends UrlHelper
{
    public function __invoke($route = null, array $params = [])
    {
        if ($this->getRouteResult()) {
            $matchedParams = $this->getRouteResult()->getMatchedParams();
            $params['locale'] = $params['locale'] ?? $matchedParams['locale'] ?? 'en';
        }

        return parent::__invoke($route, $params);
    }
}

App\Core\I18n\I18nUrlHelperFactory.php

<?php

namespace App\Core\I18n;

use Interop\Container\ContainerInterface;
use Zend\Expressive\Helper\Exception\MissingRouterException;
use Zend\Expressive\Router\RouterInterface;

class I18nUrlHelperFactory
{
    /**
     * Create a UrlHelper instance.
     *
     * @param ContainerInterface $container
     *
     * @return I18nUrlHelper
     * @throws \Interop\Container\Exception\NotFoundException
     * @throws \Interop\Container\Exception\ContainerException
     * @throws \Zend\Expressive\Helper\Exception\MissingRouterException
     */
    public function __invoke(ContainerInterface $container)
    {
        if (!$container->has(RouterInterface::class)) {
            throw new MissingRouterException(sprintf(
                '%s requires a %s implementation; none found in container',
                I18nUrlHelper::class,
                RouterInterface::class
            ));
        }

        return new I18nUrlHelper($container->get(RouterInterface::class));
    }
}

App\Core\I18n\LocalizationMiddleware.php

<?php

namespace App\Core\I18n;

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Symfony\Component\Translation\TranslatorInterface;
use Zend\Diactoros\Response\RedirectResponse;

class LocalizationMiddleware
{
    private $translator;

    public function __construct(TranslatorInterface $translator)
    {
        $this->translator = $translator;
    }

    public function __invoke(ServerRequestInterface $request, ResponseInterface $response, callable $next = null)
    {
        $acceptedLanguages = ['en', 'nl', 'fr', 'de', 'es'];
        $locale            = $request->getAttribute('locale');

        // If there is no or an invalid locale
        if (!$locale || !in_array(substr(trim($locale), 0, 2), $acceptedLanguages, true)) {
            // Try the user preferred locales
            $userLocales = $request->getServerParams()['HTTP_ACCEPT_LANGUAGE'] ?? '';
            foreach (explode(',', $userLocales) as $userLocale) {
                if (in_array(substr(trim($userLocale), 0, 2), $acceptedLanguages, true)) {
                    $locale = $userLocale;
                    break;
                }
            }
        }

        // Fallback to first accepted language
        if (!$locale) {
            $locale = $acceptedLanguages[0];
        }

        // Only the language is needed
        $locale = substr(trim($locale), 0, 2);

        if ($request->getUri()->getPath() === '/') {
            // Redirect and set the locale for the homepage
            return new RedirectResponse('/' . $locale);
        }

        // Set current locale
        $this->translator->setLocale($locale);

        // Get response first, apply content language later
        if (null !== $next) {
            $response = $next($request->withAttribute('locale', $locale), $response);
        }

        // Append the preferred language if it's different from the locale
        if ($acceptedLanguages[0] !== $locale) {
            $locale .= ', ' . $acceptedLanguages[0];
        }

        // Add the content language to the response
        return $response->withHeader('Content-Language', $locale);
    }
}

App\Core\I18n\TranslatorFactory.php

<?php

namespace App\Core\I18n;

use Interop\Container\ContainerInterface;
use Symfony\Component\Translation\Loader\ArrayLoader;
use Symfony\Component\Translation\MessageSelector;
use Symfony\Component\Translation\Translator;

class TranslatorFactory
{
    public function __invoke(ContainerInterface $container)
    {
        $config = $container->get('config');
        $config['i18n']['default_locale'] = 'en';

        $translator = new Translator($config['i18n']['default_locale'], new MessageSelector());
        $translator->addLoader('array', new ArrayLoader());
        $translator->setFallbackLocales([$config['i18n']['default_locale']]);

        $translator->addResource('array', [
            'Hello World!' => 'Bonjour!',
        ], 'fr');

        $translator->addResource('array', [
            'Hello World!' => 'Hallo!',
        ], 'nl');

        return $translator;
    }
}

App\Core\I18n\TwigTranslationExtensionFactory.php

<?php

namespace App\Core\I18n;

use Interop\Container\ContainerInterface;
use Symfony\Bridge\Twig\Extension\TranslationExtension;
use Symfony\Component\Translation\TranslatorInterface;

class TwigTranslationExtensionFactory
{
    public function __invoke(ContainerInterface $container)
    {
        return new TranslationExtension($container->get(TranslatorInterface::class));
    }
}

App\Frontend\Action\HomePageAction.php

<?php

namespace App\Frontend\Action;

use App\Core\Action\AbstractAction;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Symfony\Component\Translation\TranslatorInterface;
use Zend\Diactoros\Response\HtmlResponse;
use Zend\Expressive\Template\TemplateRendererInterface;

class HomePageAction extends AbstractAction
{
    private $template;
    private $translator;

    public function __construct(TemplateRendererInterface $template, TranslatorInterface $translator)
    {
        $this->template = $template;
        $this->translator = $translator;
    }

    public function __invoke(ServerRequestInterface $request, ResponseInterface $response, callable $next = null)
    {
        return new HtmlResponse($this->translator->trans('Hello World!'));
    }
}

composer.json

{
    "require": {
        "twig/extensions": "^1.3",
        "symfony/translation": "^3.1",
        "symfony/twig-bridge": "^3.1"
    },
}

config\i18n.global.php

<?php

use App\Domain;
use Zend\Expressive;

return [
    'dependencies' => [
        'invokables' => [
        ],

        'factories' => [
            Expressive\Helper\UrlHelper::class             => App\Core\I18n\I18nUrlHelperFactory::class,
            App\Core\I18n\LocalizationMiddleware::class       => App\Core\I18n\LocalizationMiddlewareFactory::class,
            Symfony\Component\Translation\TranslatorInterface::class => App\Core\I18n\TranslatorFactory::class,
            
            Symfony\Bridge\Twig\Extension\TranslationExtension::class =>
              App\Core\I18n\TwigTranslationExtensionFactory::class,
        ],
    ],
    'middleware_pipeline' => [
        'routing' => [
            'middleware' => [
                Expressive\Container\ApplicationFactory::ROUTING_MIDDLEWARE,
                Expressive\Helper\UrlHelperMiddleware::class,
                App\Core\I18n\LocalizationMiddleware::class,
                Expressive\Container\ApplicationFactory::DISPATCH_MIDDLEWARE,
            ],
            'priority'   => 1,
        ],
    ],
    
    'twig' => [
        'cache_dir'      => 'data/cache/twig',
        'assets_url'     => '/assets/',
        'assets_version' => null,
        'extensions'     => [
            Symfony\Bridge\Twig\Extension\TranslationExtension::class,
        ],
    ],
];

LocalizationMiddlewareFactory.php

<?php

namespace App\Core\I18n;

use Interop\Container\ContainerInterface;
use Symfony\Component\Translation\TranslatorInterface;

class LocalizationMiddlewareFactory
{
    public function __invoke(ContainerInterface $container)
    {
        return new LocalizationMiddleware(
            $container->get(TranslatorInterface::class)
        );
    }
}