Entity builder method is not invoked, plural is not saved

AFAICT, entity builders are only invoked on a request with form input, not on the initial form build. Populating the field on the initial build probably needs the #default_value to be set or similar?

@godotislate: I don't believe that is a problem, we are testing whether on form submission the entity gets the language, and it appears it does not (likely the third party setting is not saved).

@gábor hojtsy: I see now that the entity_builder is supposed to populate the third party setting on the add form, before going to the edit form in the test.

The issue about the edit form field not being populated correctly isn't an issue with the entity builder not being invoked, it's with this logic in PluralFormulaLanguageFormBase::form():

// Pick previous plural formula if editing an existing language.
    $plural_formula = '';
    if (isset($form['langcode']['#value'])) {
      $editingLangcode = $form['langcode']['#value'];
      /** @var \Drupal\language\ConfigurableLanguageInterface $language */
      $language = $form_state->getFormObject()->getEntity();
      $plural_formula = $language->getThirdPartySetting('l10n_pconfig', 'formula');
    }

In Drupal\language\Form\LanguageFormBase::commonForm(), this is how the form is built:

if ($language->getId()) {
      $form['langcode_view'] = [
        '#type' => 'item',
        '#title' => $this->t('Language code'),
        '#markup' => $language->id(),
      ];
    }
    else {
      $form['langcode'] = [
        '#type' => 'textfield',
        '#title' => $this->t('Language code'),
        '#maxlength' => 12,
        '#required' => TRUE,
        '#default_value' => '',
        '#disabled' => FALSE,
        '#description' => $this->t('Use language codes as <a href=":w3ctags">defined by the W3C</a> for interoperability. <em>Examples: "en", "en-gb" and "zh-hant".</em>', [':w3ctags' => 'https://www.w3.org/International/articles/language-tags/']),
      ];
    }

So isset($form['langcode']['#value']) in PluralFormulaLanguageFormBase::form() is FALSE, and the third party settings are not loaded to populate the form value.

That works beautifully, thanks!