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Supplementary assignment: Give examples of ways a publisher can defeat any metadata standard. Bonus points for commissioning a book that does it.
Metadata language vs Language of Text
ONIX 3.0 allows for multi-language metadata. India is a goode use case as there are 3 major languages and an audience who are multi-lingual but might be most comfortable in one language over another. In such an environment wouldn't it be great to provide the metadata in the language the reader is most comfortable in? ONIX 3.0 provides it by allowing many composites to repeat if language attributes are provided. So rather than awkwardly repeating the title three times in one entry, you provide the Distinct Title three times in separate composites set up for each language and identitifed by it's attribute. A retailer could work with such information and provide display options that could help sell books.
BookNet Canada is unaware of any North American retailer ready to use such a facility but there's no harm in having this information ready if you think it can help sell books. Before retailers implement they test and experiment – and think kindly on publishers who can support their efforts.
Summary
Don't rely on this page for all you need to know. You should go read the P.10 Language section in ONIX for Books Implementation and Best Practice Guide. It's short, practical and comprehensive, providing language knowledge without the sophomoronic humour.