Ask HN: Specialize in one language or all languages

When you work as a software engineer for a while you learn a whole list of languages: Java, Perl, Python, Ruby, JavaScript, Lisp, Clojure, iOS. I have been offering weekend programming classes in the Bay Area and I have a dilemma. Is it possible to be a full-stack trainer or does the market automatically assume you have to be a Python trainer or a JavaScript trainer? Part of the issue is that it is easier to sell training as a product when it is focused on one language, than to sell training which involves all the languages that go into a real application. Thoughts?