For now, the only supported language is Python....I wouldn't mind seeing other languages supported in GAE, though not sure it really matters or if it is worth Google's time, money, and effort to expand support to other languages. I'm sure if they asked 100 developers they would get 100 different answers of what languages should be supported.Well, just when I was getting used to the idiosyncrasies of the Python language, I went to look something up on http://code.google.com/appengine/ and found that they have an early preview of Java running on the engine. Here's the blog posting.
If I were one of the 100 developers mentioned above, my first answer would have been Java. I signed up as soon as I saw it, but alas, I'm on a waiting list. Looks like I'll be flirting with Python a little longer while I wait for my true love....
The really cool part of getting Java running in the engine is that should bring the other languages that run on the JVM as well, e.g. Scala, Groovy, JRuby, BeanShell, and, yes, even Jython.
Oh the fun!