[BUG #4926] made Demobrowser runner frame LOCALE-aware

- as config.demobrowser.json configures compile jobs from scratch, and
  compile-options/code/locales was never set
- if you want to restrict locales for the runner frame, move the top-level
  let/LOCALES to the "translation" job (where it's needed as it
  also covers the demo apps)
- and add a db-common/let/LOCALE macro, which then only applies
  to the runner frame alone