Is Duolingo enough to become fluent?

Streak apps build recognition and habit. Fluency needs spoken production, which those apps largely leave out.

Short answer: it is a fine warm-up and a poor finish line.

What it does well

Habit, streaks, and a base of recognition vocabulary. That is real value, especially at the start.

What it leaves out

Speaking at conversation speed, with your own sentences, about your own life. No tap-the-tile exercise trains that. If you want to speak, you have to spend most of your practice time speaking.