This PlanNode takes a stream of Tuples like: (ex:companyA, "Company A"@en). It assumes that the stream is sorted on
index 0 (eg. ex:CompanyA). It will cache all non-empty languages from index 1 (eg. "en") and outputs any tuples where
the language has already been seen.
If a Value on index 1 has no language because it is a literal without a language or because it is an IRI or BNode,
then its language is considered empty and not cached.