SQL is a method for accessing a database. While it is technically a language, its not completely universal. Certain databases support certain ways of querying the data, with SQL being the most common. Many databases put little twists on SQL so its SQL-like syntax but, for example, not all SQL written for Microsoft Access works with Microsoft SQL Server and vice-versa.
XML is not a language, its a DOM (Document Object Model). Its no more a language than a Word document or a picture. You use XML to create a specification for your user interface, but its not a language that you write code in.
If you want to use a database in Android, you are probably going to have to use
SQLite[
^]. I don't know of any other databases that Android supports out of the box. I'm sure there are libraries out there to support other kinds.
SQLite is easy to use, I would favor that over storing information in XML (text) files.