It's an all or nothing solution, but:
Go to Settings > General > International: And change the language to English. This will change the system language of the iPad to English. Pages will detect this and give you English templates.
Changing the language does a bit of a reset thing on the iPad, so don't panic when you see the spinning wheel for a few seconds.
I don't know of any other way.
Now, if you were to create a document from each template (while in English mode) and tuck them away in a folder (or somewhere else), you could switch the language back to Spanish and have a complete set of English templates. You'd have to remember to make duplicates when you wanted a new document, but other than that it should work fine.