Add guide to proper language negotiation

[xfq]

Fix #798.

This is a first draft. Feedback is welcome.

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[aphillips]

Don't forget about this document hanging out in development: https://github.com/w3c/i18n-drafts/blob/gh-pages/articles/language-negotiation/language-negotiation.md

There are a few things in this doc that also belong in the other. Or maybe some things in there that belong here. But we shouldn't end up with competing articles. We should have a separation of concerns if we end up with two docs.

[xfq]

Don't forget about this document hanging out in development: https://github.com/w3c/i18n-drafts/blob/gh-pages/articles/language-negotiation/language-negotiation.md

There are a few things in this doc that also belong in the other. Or maybe some things in there that belong here. But we shouldn't end up with competing articles. We should have a separation of concerns if we end up with two docs.

Here's my idea: we will have three articles related to language negotiation:

• https://github.com/w3c/i18n-drafts/blob/gh-pages/articles/language-negotiation/language-negotiation.md mainly discusses "what is language negotiation";
• https://www.w3.org/International/questions/qa-when-lang-neg discusses "when language negotiation is needed";
• this PR discusses "how to do it".

All three articles include some content on "why language negotiation is needed". What do you think of this approach? If you agree, I'll think about how to adjust these articles to minimize repetition.