Language tags & SEO

Quick question that I hope is not too far off-topic:

 

What is the role of language tags, and particularly ISO-639 codes, in SEO?

 

Asking as a Tweet by linguist Coleman Donaldson (cc'd) about limitations of
language options for SEO on a Mandenkan dictionary site he is setting up,
sounded to me like it might be related to ISO 639-3 language
categorizations. See
https://twitter.com/DonaldsonCD/status/1105583148300038144

 

Also got me thinking again re  "i+1-th level" tags. (ref.
https://www.alvestrand.no/pipermail/ietf-languages/2016-May/013336.html ).

 

Don Osborn

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