Extend Processwire TemplateEngineTwig.module with the function to enable i18n translations.

TemplateEngineTwig.module

protected function ___initTwig(Twig_Environment $twig) 
{
    $function = new Twig_SimpleFunction("__", function ($text, $filename = '', $context = '') use ($twig) {
        if (!empty($filename)) {
            if ($twig->getLoader()->exists($filename . '.twig')) {
                $textdomain = $twig->getLoader()->getCacheKey($filename . '.twig');
                $language = $this->user->language;
                $out = htmlspecialchars($language->translator()->getTranslation($textdomain, $text, $context), ENT_QUOTES, 'UTF-8');
            } else {
                $out = $text;
            }

            return $out;
        }
    });

    $twig->addFunction($function);
}