@ikaros123

Ok after some trying I was able to do it using the "t" tag on the twig template and I also modyfied the theme-settings.php.

$form['slider']['slider_code'] = [
    '#type'          => 'textarea',
    '#title'         => t('Slider Code'),
    '#default_value' => t(theme_get_setting('slider_code')),
    '#description'   => t('Please refer to this <a href="https://drupar.com/node/912/" target="_blank">documentation page</a> for slider code tutorial.'),
  ];

Now the slider_code appears in the "User interface translation" .

ikaros123 → created an issue.