sort block breaks multilingual sites

Since #1814080: Apachesolr sort block fills cache_block table without stopping has been committed, the sort block uses a custom caching.
Unfortunately the generated cache id doe not work for all multilingual settings. If you use a path prefix for language switching, everything is OK. But if you switch the language based on the domain, the cache ids are the same across different domains / languages.

The fix simply adds the current language to the cache_id.

In addition I fixed a potential warning that cache_id is not declared because it has been declared within an "if" statement.