The short answer, in general: it depends. As you correctly stated, everybody seems to have their own interpretation of what “Hispanic/latino” means, especially with regards to Brazilians.
The question asked on the Common App, though, is: “Are you Hispanic or Latino?” Since you identify as latina, nobody will fault you for checking yes.
No, learning Spanish does not make you latina 
And you can be latino/a without knowing Spanish. Also, read through the following thread:
http://talk.collegeconfidential.com/hispanic-students/641650-hispanic-latino-defined-aka-am-i-hispanic.html