The atg.endeca.index.accessor.LanguageNameAccessor class, which is a subclass of atg.repository.search.indexing.PropertyAccessorImpl, returns the name of the language that a record is in. The ATG-Endeca integration includes a component of this class, /atg/endeca/index/accessor/LanguageNameAccessor, which the ProductCatalogOutputConfig uses to obtain the value of the product.language property:

<property name="language" is-dimension="true" type="string"
  property-accessor="/atg/endeca/index/accessor/LanguageNameAccessor"
  output-name="product.language" is-non-repository-property="true"/>