- Issue created by @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" .