Drupal - How do I translate strings in Twig templates?
You need to visit the page in a language that is not English to have them picked up. That should be enough.
Use the t
filter:
{{ 'translate me'|t }}
https://www.drupal.org/docs/8/theming/twig/filters-modifying-variables-in-twig-templates
You can use the {% trans %}
Twig tag, available in Drupal 8:
{% trans %}
Submitted by {{ author.username }} on {{ node.created }}
{% endtrans %}
See the following link for more details: http://getlevelten.com/blog/mark-carver/drupal-8-twig-templates-and-translations