The multilingual index only contains language specific fulltext fields. Therfore the query needs to be rewritten even if the query is not limited to a specific language.

The name of the language field might be change in future releases of search_api. @see https://www.drupal.org/node/2641392 for details. Therefor we define a constant here that could be easily changed.

fixed Solarium\Exception\InvalidArgumentException: A filterquery must have a unique key value within a query in Solarium\QueryType\Select\Query\Query->addFilterQuery() if multiple fulltext fields are present

... to match parent declaration:
Drupal\search_api_solr\Plugin\search_api\backend\SearchApiSolrBackend::extractResults(Drupal\search_api\Query\QueryInterface $query, Solarium\Core\Query\Result\ResultInterface $result)

- we are using two dynamic fields per language now
- fix a bug in the exception messages

patch https://www.drupal.org/files/issues/2486533-6--solr5_default_operator.patch
(see also https://www.drupal.org/node/2486533)

(still commented out)

introduced naming and fallback schema for language specific fields according to https://github.com/amateescu/search_api_solr/pull/67