Strengthen form control localization requirements

[aphillips]

What is the issue with the HTML Standard?

In the I18N+WHATWG call during this year's TPAC, we discussed the localization of form controls specified in section 4.10.5.2 [here].

This issue is for tracking purposes. A PR will also be submitted that references this issue. Here's the notes from the above linked document for easy visibility:

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Addison: discussed today, some stabs at currentDir; not urgent if she's not there
… spec recommends implementation of localization of form controls in section 4.10.5.2, but this is inconsistently implemented and kinda handwavy
… would like to see implementations tighten that up to render date/time/number in a localized fashion, and strengthen to at least "should"
… can file an issue if you want

Domenic: imo, as editor, issue or PR is reasonable
… agree that it's weird that we have a non-normative/encouraged section
… would be good for the community to make it at least "should" and get implementer reasons for why they haven't implemented, or motivate them to fix it
… like this path
… section says "according to either the locale implied by…or the user's locale"

Addison: feel strongly it should follow input element's language

Anne: goes against what impls currently do, but agree that it would be better to match element language
… potentially need to ship more locale strings, because they need to support more languages

Addison: already needs to be best-effort, because you won't have all languages, so fallback applies
… probably why the "or" is there – should prefer language of element, but if unavailable the user/browser language is a good fallback