2022, UX writing course (UX Writing Hub)
This screen is part of the flow in the learning app (rather than a toast), giving the user as much time as they want to relish their accomplishment, and allowing them to show it off on social media (which could strengthen their commitment to the learning habit, like telling all your friends you’re going on a diet…)
This pop-up interrupts the flow to alert the user that they’re about to exit the session, and encourage them to come back.
This pop-up appears the minute the user hits their goal.
It comes as a nice surprise, boosting the user’s motivation and offering them an opportunity to celebrate their achievement.
To keep it interesting and a little more personalized, I would prepare a large selection of messages for this pop-up (see 10 samples on the left) and use different ones as the user progresses through their learning (starting with applauding their consistency in building the learning habit, and moving on to helping them stay on track and become lingual masters).
Hopefully the user will grow to like these messages, and wait for them to pop up with anticipation 
