So it still gave you 2 index options? Hmm.. I'm sorry it didn't do what you want, but I can't say I'm surprised. They didn't really update any of this stuff. I think it might select the correct index for you on an upgrade, tho. I'm not sure.
I believe that to update the language stuff in the Setup part (index #2) of boot.wim, only the xx-yy/WinPE-Setup*.cab (available from the ADK) are needed; since there are only es-es/* cab there, I also guess there are no differences between es-ES and es-MX there. On the other hand, the /get-langINI is required.
When you say I believe, have you actually tested what you claim or are you just guessing? I just want to spare the pain of experimenting