Interface language

The builder's own interface follows the browser's language (or the person's last choice in the top bar). An embedding app can choose it, add languages or change some words:

mountFormBuilder(el, {
  locale: 'da',                                         // 'en' | 'es' | 'da' | 'sv' | 'pt' | a code added below
  locales: { de: { name: 'Deutsch', messages: { 'topbar.undo': 'Rückgängig' /* … */ } } },
  hideLocalePicker: true,                               // the host app decides
  onLocaleChange: code => saveUserPreference(code),
});
builder.update({ locale: 'sv' });

Missing words fall back to English. To contribute a language to the project, see src/i18n/README.md and check it with npm run i18n:check.