Language specific settings for field formatters
Define a schema in your module
config/schema/mymodule.schema.yml.

Example from the date range module:

core/modules/datetime_range/config/schema/datetime_range.schema.yml

field.formatter.settings.daterange_default:
  type: field.formatter.settings.datetime_default
  label: 'Date range default display format settings'
  mapping:
    separator:
      type: label
      label: 'Separator'
      translation context: 'Date range separator'


For more information see
Block configuration form values are not translated
You need an UI to translate, which is unfortunately not ready
yet, see this issue:

Entity view/form mode formatter/widget settings have no translation
UI

Until then you can edit the translated yaml file of an exported
config in config/sync/language/[LanguageID] or install the patch from the
issue.