Devices didn't previously have a consistent way of providing the user's
current language to autofill pages. Add `.userPreferences.language` as an
optional string value for platforms to provide as part of their runtime
configurations.

The device/platform abstraction is responsible for providing a
two-character language code, and uses that language code to provide a
translation function with simple ICU-style replacements. Translations are
currently hardcoded within src/locales/strings.js, but a future commit
will extract those to JSON files in language-specific subtrees.

To enable a localized autofill experience in a language consistent with
the DuckDuckGo Privacy Essentials extension, the extension must provide
the current language code in its runtime configuration. Specifically via
the `.userPreferences.language` string property. Luckily, the extension
already provide a language code under `.contentScope.locale`, so there's
no additional plumbing required. Copy the already-present language code
to the expected location within userPreferences.

We'll be translating the entire autofill experience shortly, but those
translated strings aren't yet ready. Use translated text on a single
button (the "Use <personal duck address>" button on signup forms) to
exercise the translation flow end-to-end. Since only 'en' is supported
for translation at the moment, this has no visible change to users.

Localization will be simplest once we have all to-be-translated strings
in one place. Find all remaining hardcoded English strings and pull them
into strings.js. A future commit will automate building that file based
on downloaded language-specific JSON files.