Strange behaviour with localization and Master Pages

[Daniel Magliola]

I'm having a weird behaviour when implementing localization resource
files in my website. This looks like a .Net bug to me, but it's too
obvious to be there, so I must be overlooking something.

I have the following file structure:
+ root folder
|-----Masterpage.master
|-----Somepage.aspx
|-----App_LocalResources
| |----- MasterPage.master.resx
| |----- MasterPage.master.es.resx
|
|-----UserControls
|----- SomeControl.ascx
|----- App_LocalResources
|----- SomeControl.ascx.resx (plus it's spanish
version)

Somepage.aspx uses MasterPage.master, and in the MasterPage, some
UserControls from the UserControls folder are used.
Both the MasterPage itself, and the controls it uses show texts that
are localized.
Now, I first created the whole website in English, ready to be
localized (every text inside literal or localize controls, etc), and
now I've started creating the actual .resx files. I created the "Local
Resource" RESX files for the master page, and the user controls in it,
and it all looks great.
I open Somepage.aspx, and the "things" around it (the master page)
change languages perfectly.

The problem appears when I generate the local resource file for
Somepage.aspx. As soon as I do, all the texts in the master page and
its controls go back to English, they stop getting "localized", for
that page only.Even though there is a SomePage.es.resx, neither the
contents of the page, nor the Master Page get localized.

Again, I must be missing something very big here, but I can't find it.
If you want to see this happening, you can go to: http://peptido.dyndns.org/

The texts on the footer are on the master page itself. The ones on the
header ("Hello Guest") are in a User Control inside the Master Page.
Everything below the Blue bar, and above the footer tab is the content
of individual pages.
As you can see, by clicking on the flags on the top, texts DO get
localized (in the header and footer), no matter what page you navigate
to.
Now, if you navidate to http://peptido.dyndns.org/error.aspx (this is
the one i'm using for testing, and it's the only localized page)
you'll see that it does NOT get localized, and neither do the
surroundings.

Any help or pointer will be greatly appreciated

Thanks in Advance
Daniel Magliola