Duolingo, based in Pittsburgh, runs one of the most widely used learning apps along with the Duolingo English Test. Its engineering interviews cover coding and system design, and its experimentation culture means product thinking comes up often. This page covers what Duolingo builds and who it hires, the process candidates commonly report, the technical depth the teams care about, representative problem types, what interviewers score, and a three-week prep plan.
What Duolingo builds and who it hires
Duolingo's engineers build the learning app across Android, iOS and web, the back-end services behind lessons and streaks, the experimentation platform that tests product changes, and the English Test's proctoring and scoring systems.
- Back-end engineers: services for lessons, progress and social features.
- Mobile engineers: Android and iOS apps.
- Machine learning engineers: personalization and content.
- Data and experimentation engineers: A/B testing at scale.
What the process tends to look like
| Stage | What happens | How to prepare |
|---|---|---|
| Recruiter call | Role, team, level and timeline | Know the product and the team's area |
| Technical screen | A coding interview with a data structures or practical problem | Practical coding in your interview language, explained aloud |
| Final loop | Coding, system design for back-end roles, and behavioral and product conversations | One design in the company's domain and two strong behavioral stories |
Processes differ by team, level and office and change over time, so treat this as the common shape candidates report and confirm the specifics with your recruiter.
Coding, design and product sense
Experimentation. Duolingo is known for testing changes with A/B tests. Be ready to discuss how you would measure whether a feature helped learners.
Streaks and progress. A good design exercise is a streak and progress system that handles time zones, offline use and millions of daily updates.
Mobile quality. Mobile candidates should expect questions on app performance, offline behavior and testing.
Representative problem types
These are the kinds of problems candidates describe, not a leaked question list. Practice the category, not the exact question.
- Design a streak system that handles time zones correctly.
- Schedule review of words using spaced repetition.
- Design an A/B testing assignment service.
- Implement a leaderboard for weekly leagues.
- Optimize a slow API endpoint.
- Mobile: cache lessons for offline use.
- Medium algorithm problems.
What interviewers score
- Working code: a solution that runs, handles edge cases and is readable by the next engineer.
- Practical judgment: the trade-offs you name and the simplest design that meets the requirement.
- Product sense: understanding who the user is and what failure looks like for them.
- Ownership: examples where you shipped, measured and fixed things yourself.
The behavioral side
Technical rounds decide most outcomes, but every Duolingo loop also asks about how you work. Prepare a short story for each of these, with a result and what you learned:
- Something you shipped end to end. Cover the user problem, your decisions, what you measured and what you would change.
- A disagreement with a teammate. Show how you made the decision on evidence and kept the relationship.
- Why this company? Connect its product and engineering problems to the work you want to do next.
Structures and sample answers for the common questions are in our software engineer behavioral answers guide.
A three-week prep plan
- Week 1: practical coding: build small features against an API, parse and transform data, and write tests, in the language you will interview in.
- Week 2: system design for the company's domain, plus two behavioral stories about shipping and fixing something under pressure.
- Week 3: timed mixed problems, a mock interview every other day, and research into the product and its engineering blog.
For practice lists, use LeetCode company-tagged questions and the NeetCode 150 roadmap; for design rounds, read system design building blocks; and compare the practical style of Stripe.
Where CoPilot Interview fits
Most of a Duolingo loop is spoken: you explain an approach, defend a trade-off and talk through your past work. CoPilot Interview is an AI interview assistant whose free AI mock interview builds technical and behavioral questions from your résumé and the job description, reads them aloud and scores each spoken answer out of ten, so you can rehearse the explanation as well as the code. The free plan gives 15 minutes a day of live help in sessions of up to 15 minutes, with AI mock interviews included. Online assessments and many coding rounds forbid outside help, so practice beforehand and follow your employer's rules on the day.
Disclosure: CoPilot Interview is our product, so weigh what we say about it accordingly.
Frequently asked questions
What programming language should I use in a Duolingo interview?
Use your strongest mainstream language for general rounds; mobile roles expect Kotlin or Swift.
Does Duolingo send an online assessment?
Candidates commonly describe live coding rather than a separate online assessment, though this can vary by role and year.
How should I prepare for Duolingo coding questions?
Practice practical problems under time limits, build small features against an API in your interview language, and prepare one system design in Duolingo's domain. Explain every solution out loud, because follow-up questions about trade-offs matter as much as the first answer.
Can I use AI tools in a Duolingo interview?
Only if the instructions you receive say so. Online assessments and live coding rounds generally forbid outside help, and some companies now state their AI policy in the invitation. Use AI to practice beforehand, and follow your employer's rules.
Practice this interview before the real one
The AI mock interview builds questions from your resume and the job description, reads them aloud and scores each spoken answer out of 10. Included on the free plan.
Start a mock interview