OK - your're right I can set my portal language to en-GB to ensure my en-GB users don't see Spanish.

But I am in Spain and the primary language for my portal is Castellano es-ES.

I'm not talking about outlandish scenarios here.  OK, if I want to support en-GB (which I do for dates, currency etc) I can, however unintuative it is, set my portal language to en-GB so that my British users pickup the en-GB locale and the en-US language pack (MM/DD/YY is not obvious this side of pond).

Now I'll make a wild guess that you have your browser set as Catalan, ca.  Although I want my portal in Spanish and you probably, politics aside would prefer to see it in Castellano rather than English.  Given the current setup I cannot deliver that without a 'core mod' of deleting the base language in my locales.xml.

I'm going to quote another bit of the documentation just to show how anal I am:-

The System Locale, Default Portal Locale, and the User Selected Locale support the use
of a Fallback Locale.

Now I'm sure that is open to interpretation, but I understand it that I can specify a 'Fallback Locale' for each of the scenarios above.  If I look at the locales.xml - it's already there:-

(root)
  (language name="Español (España)" key="es-ES" fallback="en-US" /)
  (language name="English (United Kingdom)" key="en-GB" fallback="en-US" /)
(/root)

The only place it seems to be missing is in the code.

Come on, the internet's a tough enough place for non US speakers and sites.
I think that the core currently doesn't work as it should and if dynamic translation is really in the pipeline then it might good idea to set a few principals here before we take the plunge.

Solved. I just needed to disable the en-US language pack at the portal level. No need to mess with locales.xml:-)