By default, fields of type fulltext will be indexed using language-specific Solr field types. But Search API Solr doesn't provide such a language-specific filed type configuration for any language supported by Drupal. In this case the language-undefined field type will be used as fall-back. Or in case of language variations like 'de-at' the 'de' will be used as fallback. But in both cases a warning will be shown on the status report page to inform about this fact. By activating this chackbox you can suppress these warnings permanently.