How do you retain a language?

Spaced Repetition is the solution. Repeatedly learned into certain intevalles and then these come into the long-term memory. So 1 repetition after 20 minutes, 2nd after 1 hour, 3th after 9 hours, 4th after 1 day, 5th after 2 days, etc. so always longer time intervals between the repetitions and so you gradually push them into long-term memory. If you learn this information again, these are not in the long-term memory but in the medium-term memory (max. 2 months). I recommend a free card app: Anki. You can write your vocabulary on it and then they are automatically sorted in the Spaced Repetition Algorthymus and then you can learn it. (Absolute IMPORTANT: also use “Acitve Recall” if you would be repeating, you always consider for a maximum of 5-10 minutes whether this answer can still remember from memory, if max. 10 minutes around (usually they have already been retrieved) and could not retrieve, then look at it no time, but make the repetition set to 20 minutes, i.e. again from the front). This was complex, but the app “Anki” does everything, even that starts with the ones from the front, it organizes everything, or if the app “Anki” is too confusing, then “Study Smarter” also has Spaced Repi and Kartei cards.