Hi Tatsunori!
Thanks for reaching out to us! I am happy to help 🙂
If you want to know where to translate each translatable string, you should first locate the template file where the string is located. Let’s go for the “Add to cart” example:
That is a template from the Event Tickets Plus plugin. In this case, you can find the string in /wp-content/plugins/event-tickets-plus/src/views/wootickets/tickets.php in line 144.
esc_html_e( 'Add to cart', 'event-tickets-plus' );
All translation strings have a text domain that tells WordPress where to look for the translated string. In this case it’s event-tickets-plus, that means you can find the translation for that string in the Event Tickets Plus lang folder. There you will be able to locate the translation file for you own language. Example: event-ticket-plus-es_ES.po file for Spanish
You can always find the translation files for all of our plugins in here > http://translations.theeventscalendar.com/projects
Also, here is a great article on translating the premium plugins > https://theeventscalendar.com/knowledgebase/adding-or-updating-a-translation/
I hope that helps. Let me know if any other questions.
Best!
Victor