Is the command-line client the only way to submit jobs? Is there a REST or language-specific API?
This is a great and frequently asked question.
Apache Aurora does not currently have a first-class scheduler HTTP/JSON API, but it is being planned (AURORA-987) and an API is one of several ideas that has discussed as a potential projects that may be worked on during a June Aurora community hackathon.
An open call for ideas and feedback was sent out to the project mailing list a few months ago, and if you're interested in being part of that discussion I'd suggest joining #aurora on irc.freenode.net, and the project mailing list by emailing dev-subscribe@aurora.apache.org.