Solr multicore implementation uses field filters as a language configuration with a list of prioritized languages. Query can match a Content in the primary (or the most-prioritized) language, or in the one of the fallback languages, or through always available fallback.
Since each translation is indexed in a separate core, relevancy statistics for a language are kept in its core. When a secondary language is returned, it can be possible that it is scored above the matches in the primary language. Matches in secondary languages should be penalized with negative scoring factor, so that they are scored below matches in the primary language.
Penalization factor should be configurable.