Yes! You can definitely learn several languages simultaneously and you don’t need to pay anything extra for an additional course. That said, Chatterbug courses are organized in language pairs and you can only access courses that are paired with your native language. Namely:
English speakers can learn German, Spanish, and French. German speakers can learn Spanish, French and English. And Spanish speakers can learn English on Chatterbug.
You can easily add a new language to your study plan via your Settings.