How much of my personal statement should revolve around Spanish, compared to my extra-curricular activities and other subjects, if I am applying to Oxbridge?

When applying to Oxbridge specifically, from my own experience, writing about Spanish should make up the bulk of your statement, at around 80% of it. I would advise your personal statement to begin with an introduction, nothing particularly cheesy but simply outlining how your love of the language came about. Next, perhaps it would be a good idea to spend a paragraph concentrating on your academic ability (not saying your grades) and wider reading. Mention some books which have interested you, but it is imperative that you link whatever you mention to why you want to study spanish. Saying "I read X" will not convince them that you want to study spanish; instead say why what you learnt from it, why you enjoyed it, and why you are intrigued to learn more about it.
The second main paragraph should be about your interests, but still related to Spanish. So for example, in mine, my first paragraph was about reading etc, but my second concentrated on activities to show my passion: tutoring, editing the language magazine, winning a Spanish debating competition etc. If you would like, perhaps mention your other a levels at this point as say why they helped you with spanish. Finally, your last paragraph should be about extra-curriculars, but no more than 3 lines at most. They are impressive but Oxbridge are far more interested in your language. And then a conclusion to finally convince them to love you!