[manifest] Add formal defintion of language tag (closes #331) (#367)

* Fixed definition of language tag.
* Added links to ECMAS-402.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/367

-- Commit Summary --

  * Add formal defintion of language tag (closes #331)

-- File Changes --

    M index.html (39)

-- Patch Links --

https://github.com/w3c/manifest/pull/367.patch
https://github.com/w3c/manifest/pull/367.diff

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https://github.com/w3c/manifest/pull/367