You can't change the language so you'll need to go to the Spanish server and start a new account there if you want to use Spanish in the public spaces. If you're on Classic view, click on the little world icon on the top right of the page and then click on versions. That will allow you to choose Spain or any other Spanish speaking versions. You will have to make a new account there but you are allowed to have one account on as many versions as you like. The versions cant interact with each other.