How do I declare my custom entities as non-multilingual? I have written a custom module providing two custom config entities holding data, that is not language-specific. But I can't seem to find a way to tell Drupal, that there is no need to "translate" these entities - in fact, I get this action offered on every entity listing.

Even all core entities I found so far implement translation, but some provide their own translation handler. (Is this maybe a way to do it? Implementing a custom translation handler that tells Drupal not to translate anything? But how?)

Is someone out there who can point me in the right direction please?

My actual annotation look like this (please notice the "translatable" option, which is not working):

@ConfigEntityType(
  id = "ad_tag",
  label = @Translation("Advertisement tag"),
  module = "field_vimeo",
  config_prefix = "field_vimeo.ad_tag",
  admin_permission = "administer site configuration",
  translatable = FALSE,
  handlers = {
    "storage" = "Drupal\field_vimeo\AdTagStorage",
    "list_builder" = "Drupal\field_vimeo\AdTagListBuilder",
    "form" = {
      "default" = "Drupal\field_vimeo\Form\AdTagForm",
      "delete" = "Drupal\field_vimeo\Form\AdTagDeleteForm"
    }
  },
  links = {
    "edit-form" = "/admin/structure/field_vimeo/ad_tags/manage/{ad_tag}",
    "delete-form" = "/admin/structure/field_vimeo/ad_tags/manage/{ad_tag}/delete"
  },
  entity_keys = {
    "id" = "id",
    "type" = "type",
    "label" = "label"
  },
  config_export = {
    "id" = "id",
    "type" = "type",
    "label" = "label",
    "data" = "data"
  }
)
Translatability for config and content entities is different. Translation handler is a content entity thing. Storage for translations needs to be explicitly enabled by adding a data table.

For config entities, it's different. Translatability works automatically there through config overrides. There's nothing that you need to do to opt-in to being translatable, but you also can't prevent it. What you can is control which properties are translatable through the config schema. Like the label,which is, technically, a translatable, user-provided string.

If you don't need it translated, just don't create any translations for it  or if you don't need any configuration translations, don't enable the config translation module.