Enable translation for theme settings
in schema change field type to text and add translatable: true

mytheme.settings:
  type: theme_settings
  label: 'My theme settings'
  mapping:
    custom_textarea:
      type: text
      label: 'Additional settings'
      translatable: true


You may also face this problem later https://www.drupal.org/project/drupal/issues/2719553 (my reply there is the last from current date)