Create filter by language that can be applied to objects before passing to serialization. Intended to be used before WikibaseDataModelSerialization. This is to replace the option on the current serialization for filtering by language.
FilterByLanguage will reside in WikibaseDataModel. It has as input an EntityDocument that has a Fingerprint, an AcceptLanguageSet, a Transliterator or something like that. It has as output the new EntityDocument.