Customizer: the preview UI language should be the user language

[afercia]

In WordPress 4.7 there are two languages (the site language and the user language) and any controls, text, messages, etc., that are part of the UI should be displayed in the user language, i.e. the one users choose for the admin interface.

By the way, the Customizer preview UI uses the site language. See the screenshots below, taken using these settings:

User language: Spanish
Site language: Deutsch

For example, the edit buttons title and aria label attributes should be in Spanish, as well as the "Shift Click" tooltip:

[celloexpressions]

If the customize preview is running in the wrong language, the scope of the problems could be much larger than edit shortcut aria labels. There may be additional challenges here, though, because the theme is presumably using the site language. There needs to be a way to detect "admin" strings that are running in the theme/frontend context.

[ocean90]

39009.patch​ makes use of switch_to_locale() and get_user_locale().

[westonruter]

In 39545:

Customize: Use selected user language for edit shortcuts in preview instead of site language.

Props ocean90.
Fixes #39009.

[westonruter]

Re-opening for 4.7.1.

[dd32]

In 39571:

Customize: Use selected user language for edit shortcuts in preview instead of site language.

Props ocean90.
Merges [39545] to the 4.7 branch.
Fixes #39009.