Add custom (disabled) languages

Here is a patch that solve this issue:

1-Most of the patch is made of a new ConfigEntityBase named "CustomLanguage": it will store the added languages and it will provide forms to edit, update or delete the languages.
I will also provide a list view of the added language at admin/config/languagefield

2-The added languages are merged with the "core" languages with those lines in LanguageItem.php:
a)Adding the custom languages at construct time (stored in $customLanguages):

  protected $customLanguages;

  public function __construct(DataDefinitionInterface $definition, $name = NULL, TypedDataInterface $parent = NULL) {
    parent::__construct($definition, $name, $parent);
    $storage = \Drupal::entityTypeManager()->getStorage('customlanguage');
    $lang_ids=$storage->loadMultiple();
    foreach ($lang_ids as $cust_lang) {
      $this->customLanguages[$cust_lang->getLangCode()]=[$cust_lang->label(),$cust_lang->getLangNativeName()];
    }
  }

b)Add custom languages in the options (one line added)

  public function getPossibleOptions(AccountInterface $account = NULL, $format = 'en') {
    $select_options = array();

    // No need to cache this data. It is a hardcoded list.
    $standard_languages = \Drupal::languageManager()->getStandardLanguageList();
    
    //Add the custom languages to the list
    $standard_languages+=$this->customLanguages;
    ....

c)Fix the getNativeName function (one line added)

  public function getNativeName() {
    $standard_languages = \Drupal::languageManager()->getStandardLanguageList();
    $standard_languages+=$this->customLanguages;
    return $standard_languages[$this->value][1];
  }

3-The patch adds also a permission to view, edit, create or delete the custom language: "CRUD languages for languagefield"

4-The patch adds a menu item under "system.admin_config_regional" to see the custom language list

5-The patch adds a schema.yml file to allow the creation of custom languages in the form of yml text files

Let me know if you think that this patch goes in a right direction.

Note: I am not sure the exact process to make this patch working, but here is my idea:
1-Clear the cache (drush cr)
2-Update entities (drush entup)

I have added 2 more changes in this patch:
1-Add a getLanguage() public function in LanguageItem class

  public function getLanguage() {
    $langcode=$this->value;
    $language=ConfigurableLanguage::createFromLangcode($langcode);
    if (array_key_exists($langcode, $this->customLanguages)){
      $language->set("label", $this->customLanguages[$langcode][0]);
    }
      return $language;
  }

This function is getting the language "the normal way" through createFromLangcode, and if the language code exists in the customLanguages list, it will update the label property with the name of the custom language.

2-Update the FieldFormatter viewValue function:
I change:
$language = ConfigurableLanguage::createFromLangcode($langcode);
into

//Must call LanguageItem::getLanguage to have the benefit of added custom languages
$language = $item->getLanguage();

In other word, I used the newly added function to get the language object

I found a small error in:

  public static function getCustomLanguageList() {
    $storage = \Drupal::entityTypeManager()->getStorage('custom_language');
    $lang_ids = $storage->loadMultiple();
    foreach ($lang_ids as $language) {
      $languages[$language->id()] = [$language->label(), $language->getNativeName()];
    }
    return $languages;
  }

We should add

$languages=[];

Before the foreach in the case there isn't any language enoded.