Don’t make the fallback locale the only one active

If every locale is implicitly active (`$config['active_locales']` is empty) the fallback locale doesn’t need to be added. Even worse, it populates the active_locales list and thus deactivates the other locales.

So: Only add it if it’s really missing.
Merge pull request #121 from corphi/patch-1

Don’t make the fallback locale the only one active