"Global: Custom text" field translation
In Drupal 8 and 9, use t ( or trans) filter in twig element in your Global: Custom text”.
for example, create a Global: custom text field and paste
 {{ 'this text will be available in translate interface'|t }}

Ref for reading more: https://www.drupal.org/docs/theming-drupal/twig-in-drupal/filters-modifying-variables-in-twig-templates