Custom field type: make only one specific property translateable

Question

Is it possible to mark only a single property of a field as translateable?

E.g. I have a custom field type RoomRates with 2 properties uri and price_mode.

/**
 * Plugin implementation of the 'kognitiv_room_rate' field type.
 *
 * @FieldType(
 *   id = "kognitiv_room_rate",
 *   label = @Translation("Kognitiv room rate"),
 *   description = @Translation("Stores an URL to retrieve price and availability data for a room"),
 *   default_widget = "kognitiv_room_rate",
 *   default_formatter = "kognitiv_room_rate",
 *   cardinality = 1,
 *   column_groups = {
 *     "uri" = {
 *       "label" = @Translation("URI"),
 *       "translatable" = TRUE
 *     },
 *     "price_mode" = {
 *       "label" = @Translation("Show prices as")
 *     }
 *   }
 * )
 */
class RoomRate extends FieldItemBase implements LinkItemInterface {

  const PRICE_MODE_PERSON = 'PERSON';
  const PRICE_MODE_DAY = 'DAY';

  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $properties['uri'] = DataDefinition::create('uri')
      ->setLabel(t('URI'))
      ->setRequired(TRUE);

    $properties['price_mode'] = DataDefinition::create('string')
      ->setLabel(t('Show prices as'))
      ->setRequired(TRUE);

    return $properties;
  }

  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    return [
      'columns' => [
        'uri' => [
          'description' => 'The URI of the JSON API endpoint.',
          'type' => 'varchar',
          'length' => 2048,
        ],
        'price_mode' => [
          'description' => 'Show prices as price per day or price per person in std occupancy',
          'type' => 'varchar',
          'length' => 10,
        ],
      ],
      'indexes' => [],
      'foreign keys' => [],
    ];
  }

The uri property should be translateable (if the site builder enables translation for this field), while the price_mode property should never be translateable/always be the same accross all languages (no matter if the site builder enabled field translation or not).

Answer 1

content_translation has a feature that synchronizes certain property between translations, see how the \Drupal\image\Plugin\Field\FieldType\ImageItem field type uses that:

 *   column_groups = {
 *     "file" = {
 *       "label" = @Translation("File"),
 *       "columns" = {
 *         "target_id", "width", "height"
 *       },
 *       "require_all_groups_for_translation" = TRUE
 *     },
 *     "alt" = {
 *       "label" = @Translation("Alt"),
 *       "translatable" = TRUE
 *     },
 *     "title" = {
 *       "label" = @Translation("Title"),
 *       "translatable" = TRUE
 *     },
 *   },

By not setting translatable = TRUE, the UI by default suggests to keep that value across translations, although it can be configured differently.