I am now trying to translate the minimap icons... I edited them in Paint but it messed up the transparency so now I'm redoing it in Photoshop.
Use Paint.net. Just as simple as MS Paint, but with greater tools such as transparency. Microsoft needs to step up their game. They're late by about 15 years in multiple areas.
I have already done it with Photoshop CS6, and it shows perfectly in-game. The most tricky part was changing the tiles' location in the image, and their width (this can be done by editing the .xml files).
The only texts that lack translation are some strings that seem to be in the .exe or somewhere else where they can't be easily modified. If I remember correctly, they are 'Player #' (the generic username when playing in a local game) and the modifier strings (No modifiers, # challenges, # clutches). The official map names and descriptions, too, but those are in the .hwm files and editing them directly cause the maps to become unusable.
Here's the translation files, if anyone wants to take a look:
http://dl.dropbox.com/s/vmwerf4dkadon7n/Hammerwatch%201.23%20en%20castellano.rar