After searching a while, I started to play with a nice but somewhat old freeware called BK ReplaceEm. Its a multiple search and replace utility, able to save different sets of phrases.
I took a copy of index.php and added ## at the start and end of everything needing translation to build my english to spanish set.
In fact there aren't too many items to translate, but it seems a quick way to re-translate the file on every new release. If you like the idea, I'll upload a rar with everything inside once I finish.
The "translation set" would also serve to create the language file I suggested before (translating through variables)