I have done the same thing as you are describing, but I started a new character using a german language pack on a german server.
The results were not satisfying. You wont learn proper language from 12-17 year olds, I can tell you that much. You might be able to pull
it off by sitting next to a spanish-english wordbook most of the time (but think of all the slang that young people uses), this will be no easy task.

I wish you good luck, but don't transfer off your character just yet, try and create new ones and level them to 30~ and see if you can increase your vocabulary.