Boosting is limited to one boost per field

I have created a patch for this issue.
I was then able to make a custom processor to use preprocessSearchQuery to add in the multiple boosts. My example:

  public function preprocessSearchQuery(QueryInterface $query) {
    parent::preprocessSearchQuery($query);

    // Get the current user's current language.
    $current_language = $this->languageManager->getCurrentLanguage()->getId();

    // Define the language priority order.
    $language_priority = [
      'en' => 6,
      'fr' => 5,
      'es' => 4,
      'ar' => 3,
      'zh-hans' => 2,
      'ru' => 1,
    ];

    $language_priority[$current_language] = 8;

    // Add a boost based on the langcode field.
    foreach ($language_priority as $langcode => $boost) {
      $query_boost[$langcode] = sprintf('if(eq(%s,"%s"),%2F,0.0)', SolrBackendInterface::FIELD_PLACEHOLDER, $langcode, $boost);
    }

    // Works now with search_api_solr_multiple_boost_per_field_3492868_3.patch.
    $boosts['search_api_language'] = $query_boost;
    $query->setOption('solr_document_boost_factors', $boosts);
  }

I don't think that we should add that workaround to search_api_solr.
The feature request should target search_api first to allow multiple boosts.
Then the backends should be adjusted accordingly.

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