function updateI18n() {
  return gulp.src(paths.templates)
  .pipe(plumber(logError))
  .pipe(i18n.extract({
    base: 'src/_locales',
    manifest: 'src/manifest.yml',
    touchedOnly: false,
    useDefaultLang: false,
    markUntouched: true,
    extension: '.yml',
  }))
  .pipe(gulp.dest('src/_locales'));
}
Load locale files (src/_locales/<lang>/message.[json|yml]), and
update them with keys in template files, then store in `message.yml`.