Language specific settings for field formatters I've created a custom field formatter widget for numeric fields. The formatter has A LOT of settings to customize the visual output of that number, to whatever designers/marketing wants (adding a lot of HTML markup, with[out] leading/trailing zeros, zeros as dash, one or two fractions, rounding, prefixes/infixes/suffixes,... ).  Some of those settings are language-dependant though, so I'd like to enter different formatter settings in

/en/admin/structure/types/manage/MY_CONTENT_TYPE/display
/de/admin/structure/types/manage/MY_CONTENT_TYPE/display
/fr/admin/structure/types/manage/MY_CONTENT_TYPE/display

I've noticed that there is a $langcode parameter in
FormatterInterface::viewElements(FieldItemListInterface $items, $langcode)

but I don't know how to make
FormatterInterface.php FormatterInterface::settingsForm(array $form, FormStateInterface $form_state)
language-aware.

Is it possible at all to have language-specific settings in a field formatter? If yes, how to I store and retrieve those settings in the correct language?
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.