Why this specific string is untranslatable
There are two different aspects. The first one is how Drupal compiles the core translation list. This is based on the (not running) PHP code using the Translation template extractor (POTX).

You find only the first of the four messages on localize.drupal.org because of a naming issue in this particular constraint. Drupal searches for translation strings in certain locations and $messageParent doesn't match:

/**
 * Detect validation constraint messages. Drupal 8+.
 *
 * This sequences is searched for:
 *   T_POTX_CONSTRAINT = T_CONSTANT_ENCAPSED_STRING.
 *
 * note: T_POTX_CONSTRAINT is marked for T_VARIABLE tokens inside .php files
 *   with the "Constraint" suffix, where the token is "$message", or ends
 *   with "Message"
 *
 * @param string $file_name
 *   Name of file parsed.
 * @param string $save_callback
 *   Callback function used to save strings.
 */
function _potx_find_constraint_messages($file_name, $save_callback) {


See https://www.drupal.org/project/potx/issues/2884361#comment-13052396

The second aspect is about the run-time behavior. Symfony contraints are integrated into the Drupal translation system, so that messages are wrapped in a Drupal translatable markup object.

So a quick fix would be to visit the site in a non english language and trigger the error, then the string will be added to the translation list of your local site.