Only official Canon firmware updates can and should be used to update firmware: who else knows
which safety checks to make and (maybe) gives you your money back if it goes wrong ?
CHDK is 100% RAM based, and never writes to the flash ROM, so it can not help you with this.
If Spanish on your cam is *really* important for you, you could 'speculate' with it though:
sell it (now that it is still relatively a new model) for as much as you can get and wait till the
last moment before you go to S-America to buy a new one - preferably in Spain, because
with joblessness at 14% there now, i would think deflation can't be far off. Add to that the
'normal' tendency of prices to fall as cams are further away from introduction, and hey,
you could even come out with a small profit
[disclaimer: just thinking out loud here, but in the crazy times we live in $$$-wise, it might even work
]
sorry to be the bearer of bad news,
wim