Improve Locale support

[JedWatson]

We've added initial support for interface localisation but it's incomplete / broken for non-English users, specifically:

• Not all strings are localised so the UI is a mix of English and the Browser's locale
• The translated strings are probably wrong, they're currently sourced from Google Translate
• You can't override the Locale

To make this less broken we should do a few things:

• [x] Default the Locale specifically to English and make translations opt-in so people can test them (also allow them to be set in config) #402
• [ ] Document the current state and ask for help translating strings
• [ ] Make all the strings locale-based, removing the hard-coded English parts

Later when we have more confidence in our translation support, we could (re)introduce the option to use the browser's locale so that implementations of Keystatic with users who have different locales see their own, but right now that would lead to a broken experience on a per-end-user basis so let's leave it with the config to set globally for each project.