"Julien est en train de parler."
Translation:Julien is speaking.
The French language lacks a present progressive for its verbs, and thus in the present tense, "il mange" can mean both, "He eats", and, "He is eating", when translated to its English equivalent. To emphasize on an ongoing/continuous state of the verb, the phrase "être en train de" can be helpful in expressing this tense. "Il est en train de manger" can thus be best translated into: He is (in the process of) eating.
Also interested, i found some examples here: http://context.reverso.net/traduction/francais-anglais/je+suis+en+train+de+parler
Based on that link, I would say that our sentence is missing some context, so we are right to assume that 'Julien parle' is also adequate. Duo is trying to teach us a new word/expression.