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

> by catering to 1 by inheriting by default, we hurt 2

2 can either be good or problematic, but I think "good" cases win over "problematic" cases.

1. Inheriting is problematic when showing English text in an editor component in a French page because spellcheck thinks it's French.
2. Not inheriting is problematic when showing non-English text in an editor component in any pages because spellcheck thinks it's English when lang is not set.

1 is predictable and I can afford to, that happens when I pasted different languages into my plain text editor too.

2 looks hopeless. All such language dependent components would not be usable outside of English unless component authors explicitly support getting lang from the shadow host.

Spellcheck for one example, but there are others, such as underline or [text emphasis](https://drafts.csswg.org/css-text-decor-3/#text-emphasis-position-property). I can accept Japanese text is underlined like Chinese if the page is Chinese, but all text in shadow trees in any pages being Chinese isn't really acceptable.

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