Subfield $b of the 040 field (Language of cataloging) contains a code for the language used in cataloging; i.e., the language used in notes, subject headings, etc. Do not confuse this with the language of text (008/35-37) in the bibliographic record. Apparently there is a misunderstanding about this code. In bibliographic records issued by the Library of Congress, regardless of source, there should either be no 040 $b or if one is retained it should only contain the code for English (eng).
Note that for authority records, LC began using 040 $b in 1998 (cf. Authority Data Elements Implementation posted on the CPSO Home Page). But again, the only valid code is "eng".
Kay Guiles CPSO
Library of Congress Guidelines for the treatment of copy cataloging records for books with language of cataloging other than English.