i18n

In the Twig templates, setup the text like so

{% trans 'Hello world' %}

To generate the POT file, run gentrans in the tools directory. That will regenerate the messages.pot file in the locale directory.

Use POEdit to open a message.po file in ROOT_DIR/locale/{locale}/LC_MESSAGES/

Then run Catalog->Update from POT file and select the messages.pot file in the root of the locale directory.