Quick Answer
Duolingo is a free, game style app that builds a daily English habit through short lessons, with an optional Super subscription and a one week free trial. Mockly is paid live coaching, $50 an hour, where an engineer runs a coding, system design or behavioural round in English. Start with Duolingo for basics; switch to Mockly when interviews are near.
What Duolingo Is Built to Do
Duolingo is the app most people have tried at least once. Its homepage, checked on 24 October 2026, describes it as “free, fun, effective”: quick, bite-sized lessons in which you earn points and unlock levels, with courses that teach reading, writing, listening and speaking.
Facts from duolingo.com
- The core app is free.
- Lessons are personalised with AI to your level and pace, according to Duolingo.
- Game-like features, challenges and reminders from Duo the owl are designed to keep you coming back every day.
- Super Duolingo is an optional subscription with a 1-week free trial. Duolingo says you can cancel any time up to 24 hours before you are charged, and Super Family covers up to six accounts.
- The Duolingo English Test is a separate product: an online English certification, not a course.
This makes Duolingo very good at one thing that matters a lot for language learning: consistency. Ten minutes a day for a year adds up, and the streak mechanics make it easy to keep going.
Insight
Duolingo's Super page did not show a price when we checked; it is shown inside the app and can vary by country. We do not quote one here.
The Gap Between App English and Interview English
A tech interview in English needs a very specific kind of speaking. You talk continuously for 45 minutes or more, mostly about technical decisions, while someone asks follow-up questions you did not prepare for. Short app exercises are not designed for that, and that is not a criticism of Duolingo; it is simply a different goal.
Here is what the difference looks like in practice:
Four things a tech interview tests that short lessons rarely do
- Sustained speaking. Talking for several minutes without a break while also thinking about the problem.
- Recovering from mistakes. Saying “sorry, let me rephrase that” and carrying on, instead of stopping.
- Being interrupted. Answering a follow-up question in the middle of your explanation, then finding your place again.
- Numbers and units. Saying “fifteen milliseconds” or “fifty thousand requests per second” clearly. The teen vs ty mistake is a classic example.
None of these sentences is grammatically hard. What makes them hard is producing them in real time, in the right order, while you are also solving the problem. That skill comes from rehearsing the situation itself, with someone who can stop you and say “that part sounded unsure; try it like this”.
A Four-Stage Path from Duolingo to Your First Tech Interview
Most engineers do not need to choose between an app and coaching. They need the right one at the right stage.
Interview tip
If you are not sure whether you are past Stage 1, try saying your current job, team and one achievement out loud for 60 seconds. If you can do that without stopping, you are ready for interview practice.
Free Mini App
Check your tech interview English for free
Free, no card needed. Practise the spoken English of a tech interview before you book anything.
Try the Mini AppCost: Free App vs Paid Coaching
The price gap is real and worth being honest about. Duolingo's core app is free, and Super is optional. Mockly's live coaching is not free beyond the 30-minute trial:
- Mockly: a free 30-minute trial with a coach on Google Meet. No card and no subscription.
- Mockly: live 60-minute 1:1 sessions are pay-as-you-go. One session is $50; packs of 5 ($237.50), 10 ($450) or 20 ($850) cost less per session. Credits last 12 months and work with either coach.
- Mockly: Premium is the self-study course at $9.99/month, or $59.99/year, with a 7-day free trial. It does not include a coaching session and you do not need it to book one.
So the question is not which is cheaper; Duolingo clearly is. The question is what each hour is worth to you. If a job offer depends on how you perform in a few interview rounds, a small number of targeted sessions may be worth more than months of general practice. If you are years away from interviewing in English, spending money on coaching makes little sense yet.
Insight
A useful rule: spend nothing on interview coaching until you have interviews in sight. Then spend on the rounds you will actually face.
Quick Self-Check: Are You Ready to Move Past the App?
You are probably ready for interview practice if you can…
Stay with an app a little longer if…
Mockly's coaches say B1 conversational English is enough to start. They do not run accent drills; they slow down, rewrite sentences you can actually say under pressure, and raise the bar as you get more fluent.
Verdict: Duolingo or Mockly?
For other app comparisons, see Mockly vs Speak and Mockly vs ELSA.
Talk to an engineer-coach first
Book a free 30-minute Google Meet with Sophie or Tom. No card, no subscription. You practise a short interview-style prompt in English and leave with a plan for what to work on next.
Book a Free Trial →