Appendix 2. Declare the language of the resource at the item level: Implementation examples following metadata standards/guidelines

Datacite Schema 4.49 LanguageUsage: optional Occurrence: 0-1 (Not repeatable)Recommended encoding IETF BCP 47 or ISO 639-1 language codes
Dublin Core (DC)Term Name: languageUsage: optionalOccurrence: repeatableRecommended practice is to use either a non-literal value representing a language from a controlled vocabulary such as ISO 639-2 or ISO 639-3, or a literal value consisting of an IETF Best Current Practice 47 [IETF-BCP47] language tag.
Electronic thesis and dissertation metadata standard (ETDMS)dc.languageUsage : Optional,Occurrence: 0-N (Repeatable) Language names themselves should be recorded using ISO 639-2 (or RFC 1766). If the language is not specified, it is assumed to be english (en).
Metadata Object Description Schema (MODS)Top Level Element <language>Usage : OptionalOccurrence: 0-N (Repeatable)This resource contains both English and French text:<language><languageTerm type=”code” authority=”iso639-2b”>eng<languageTerm></language><language><languageTerm type=”code” authority=”iso639-2b”>fre<languageTerm></language>
This resource contains text in Egyptian Arabic, which is coded as an individual language in ISO 639-3:<language><languageTerm type=”code” authority=”rfc4646″ >zh-Hans</languageTerm></language><language><languageTerm type=”code” authority=”iso639-3″ >arz</languageTerm></language>
OpenAIRE Guidelines for Literature, institutional, and thematic Repositoriesdc:languageUsage: Mandatory if Applicable (MA)Occurrence: 0-N (Repeatable)Recommendation: take values from one of the following lists:IETF BCP 47, the IANA Language Subtag RegistryISO 639-x, where x can be 1,2 or 3. Best Practice: we use ISO 639-3 and by doing so we follow: http://www.sil.org/iso639-3/If necessary, repeat this element to indicate multiple languages.If ISO 639-2 and 639-1 are sufficient for the contents of a repository they can be used alternatively. Since there is a unique mapping this can be done during an aggregation process.
Japan Consortium for Open Access Repository (JPCOAR)dc:languageUsage: R (Recommended)Occurrence: 0-N (Repeatable: expect mandatory term)Usage InstructionsEnter the main languages that are used in the main text of the resource. Use the ISO 639-3 language codes.It is optional to use the ISO 639-3 macrolanguage.NotesDo not enter language names.Do not enter country names.Enter in order of language priority.Recommended ExamplesThe main text of the resource is in English.<dc:language>eng</dc:language>The main text of the resource is in English and Japanese.<dc:language>eng</dc:language><dc:language>jpn</dc:language> Unrecommended ExamplesISO 639-1 is not recommended.<dc:language>ja</dc:language>
Do not enter multiple languages in one element.<dc:language>engjpn</dc:language>Do not use capital letters and double-byte characters.<dc:language>JPN</dc:language><dc:language>eng</dc:language>Do not enter language names.<dc:language>日本語</dc:language>Do not enter country names.<dc:language>US</dc:language>Do not enter language codes other than ISO 639.<dc:language>en_US</dc:language>

Source: https://comments.coar-repositories.org/appendix-2-declare-the-language-of-the-resource-at-the-item-level-implementation-examples-following-metadata-standards-guidelines/