Overwrite protection lost for plural formulas due to dead code

Problem/Motivation

In 2010 in Drupal 6 I introduced protection for plural formulas already on the site in #566390: Apply locale import protection to the plural forms unless explicitly requested to overwrite translations and the plural formula. Unfortunately due to dead code this feature was lost in Drupal 8. PoDatabaseWriter::setHeader() currently has this check:

$locale_plurals = \Drupal::state()->get('locale.translation.plurals', []);
...
if (array_sum($overwrite_options) || empty($locale_plurals[$langcode]['plurals'])) {
  // Get and store the plural formula if available.
  $plural = $header->getPluralForms();
  if (isset($plural) && $p = $header->parsePluralForms($plural)) {
    [$nplurals, $formula] = $p;
    \Drupal::service('locale.plural.formula')->setPluralFormula($langcode, $nplurals, $formula);
  }
}

Unfortunately this uses the wrong state key, so while it intends to protect the overwrite it does not work.

Since #2571375: Remove TranslationManager dependency from LanguageManager (Drupal 8.0.0), PluralFormula stores the plural formulae in the state locale.translation.formulae. Core does not set locale.translation.plurals any more, so $locale_plurals is always empty, and every import replaces the language's plural formula. This includes imports with "Overwrite non-customized translations" and "Overwrite existing customized translations" off, and drush locale:import --override=none.

locale_uninstall() also deletes the nonexistent locale.translation.plurals, not locale.translation.formulae. So uninstalling Locale leaves the plural formulae in the state.

This combined with the exported files always getting one hardcoded plural formula of nplurals=2; plural=(n > 1); means that even if you already had the plural formula properly set up, importing an exported translation back will break your plural setup. Core does not even have a UI to fix this or any UI that tells you something is broken now.

Steps to reproduce

1. Add the Hungarian language.
2. At Administer » Configuration » Regional and language » User interface translation » Import, import a Hungarian .po file with "Plural-Forms: nplurals=2; plural=(n != 1);\n" in its header.
3. Import a Hungarian .po file with "Plural-Forms: nplurals=3; plural=n==1 ? 0 : n==2 ? 1 : 2;\n" in its header, with "Overwrite non-customized translations" and "Overwrite existing customized translations" off.
4. \Drupal::service('locale.plural.formula')->getNumberOfPlurals('hu') is 3. It should be 2.

This is even worse if the file does not have a plural formula or has an invalid plural formula.

Proposed resolution

Properly check if the language already has a plural formula and protect it where we should.

Remaining tasks

Implement the fix.

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

None.

LLM disclosure

LLM was used to find and fix this issue. With human review.