If you look at the English department in your school, you'll find that "English" is a broad category that usually includes English grammar, how to write formal research papers and essays, and understanding literature that has been written in English. Even public speaking and performing arts often fall under the umbrella of "English." It may seem too obvious, but the common thread among all of these topics is that they all have to do with using the English language.
Sometimes (especially at the college level), you may take a class that focuses on just one aspect of English — such as, American Literature or Expository Writing — but when you find yourself in a class called something like Senior English, chances are it's going to contain a mixture of all of the topics listed above.
It does seem strange to call a class about novels and plays "English," but whether you're aware of it or not, understanding the language is a huge part of all English classes. Think about the last time you read Shakespeare. Wasn't it sort of like learning a new language? Even when you study more modern pieces of literature, your brain is soaking up loads of new words and new ways of describing the world. So, you are, in that sense, continuing to learn the language with every English class that you take.
In contrast, your other language courses like Spanish or French probably focus on grammar and vocabulary, which makes them seem very different from your English class right now. (After all, you're probably starting from scratch with these languages!) But, if you continue to study a second language for more than a few years, the focus of the curriculum will soon shift to reading literature and writing essays in those classes, too!