Refer to BCP 47, not to RFC 5646

[xfq]

This PR uses the recommended URL of the IETF language tag standard, so if the spec is updated, we don't need to update the URL. This is also the URL recommended by W3C (for many years):

The link to and name of BCP 47 was created specifically so that there is an unchanging reference to the definition of Tags for the Identification of Languages. RFCs 1766, 3066, 4646 were previous (superseded) versions and 5646 is the current version of BCP 47.

In terms of content, RFC 5646 is just the core of BCP 47, so it doesn't introduce or remove any language tags. (The language subtags are managed by the IANA Subtag Registry.)

[github-actions]

Preview URLs

• /en-US/docs/Web/HTTP/Headers/Content-Language

External URLs (4)

URL: /en-US/docs/Web/HTTP/Headers/Content-Language
Title: Content-Language

• https://en.wikipedia.org/wiki/ISO_639 (1 time) (Note! This may be a new URL 👀)
• https://en.wikipedia.org/wiki/Language_code (1 time) (Note! This may be a new URL 👀)
• https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes (1 time) (Note! This may be a new URL 👀)
• https://www.rfc-editor.org/rfc/bcp/bcp47.txt (1 time) (Note! This may be a new URL 👀)

[wbamberg]

👍 thank you, @xfq !