Two primary risks, both identified through experience rather than theory.
First, building without validation. The product must be validated component by component as each milestone is completed, not at the end. If the alphabet lesson is built, it should be in front of real learners now. If greeting dialogs are built, they get tested immediately. Every unvalidated component is accumulated risk — time invested in something that might not work, discovered too late to course-correct cheaply. The discipline is: build a milestone, test it with real users, refine it based on what you learn, then build the next one.
Second, sustained pace. Real users hitting real milestones provides concrete evidence of progress that sustains momentum. Building in isolation without feedback makes it harder to maintain energy and direction. Seeing someone else learn the Russian alphabet in a day using your system reinforces that the work matters and clarifies what to build next. The validation discipline and the momentum problem solve each other — each validated milestone is both product progress and fuel for the next phase.