Re: [webcomponents] lang and xml:lang should not use the flat tree (#385)

Yeah, right, for the same reason as `dir`, I think `lang` should inherit.

These two attributes are more environmental, affects text rendering and font fallback even into form fields today in most browsers. Think they're the language setting in your OS which can reach deep into date controls in applications. Or when your app changes the locale of the process, it affects all locales in 3rd party controls in your app.

Also, technically, `dir` is mapped to the CSS `direction` property, and CSS inherits composed tree, so `dir` and `direction` behaving differently is problematic. Blink and WebKit maps `lang` to CSS `-webkit-locale` property so the same technical constrain applies, though it's not a standard property.

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https://github.com/w3c/webcomponents/issues/385#issuecomment-185645716