The MARC exporter incorrectly populates the 040$b field (Language of cataloging) using the language of the material, rather than the language of cataloging. Until "Language of description" is mapped to the ISO 639-2b language set dropdown, the 040$b should either be set to "eng" or left blank.
Work is happening now on adding a Language of Description area that will make this possible. See https://archivesspace.atlassian.net/browse/ANW-382. The export will be corrected at that time. We expect to have this in a winter release of ArchivesSpace.
Closed because addressed with the update of the exporter as part of the languages work.