Required field.

3-letter codes, lower-case, indicating the language versions in which a document or publication is available or issued.

See CODES for UNBIS language codes used in field 041.

Codes for language versions are recorded in a continuous string; codes for official UN languages are recorded first, in alphabetical order, followed by codes for non-official languages in alphabetical order.
In case of multilingual documents, the 3-letter code mul should precede the language codes.

If a document is only issued in certain languages, and the document series is normally issued also in other languages (this is usually specified on the document masthead below the symbol and imprint as «ENGLISH ONLY», «FRANÇAIS SEULEMENT», etc.) then a 546 Language Note is added : «English only», «French and Spanish only», etc. corresponding to the language codes entered in 041.  

If a document contains summaries in various languages in addition to the language of the publication, those language codes are specified in 041 $b.

Examples:

041 $a arachiengfrerusspa
[Document issued in all official languages]

041 $a mulengfrerusger
[Multilingual document: English, French, Russian, German]

041 $a frespa
546 $a French and Spanish only.
[Document issued only in French and Spanish, which will not be issued in other language versions]

041 $a eng $b frespa
[Document in English, includes summaries in French and Spanish]