🌎 Explicitly set `lang` (language) attribute in J-Editor and on rendered post content

[robin-macpherson]

Perceived Problem

• Some users still report issues typing accented characters and other IME-related input

Ideas / Proposed Solution(s)

• One thing that may help with this is setting the lang attribute on the editor and post content
• This will require storing a mapping to BCP47 language tags for each language that we support and then setting the attribute value based on the language the user selects

Bonus

• It would be nice to set the top-level html element's lang attribute to be the user's UI language as well, mostly for screenreader users