If you're serious about language learning, I believe you should dedicate at least 30 minutes a day to your target language, if not more.

But sometimes life gets in our way, and we have more urgent things to deal with.

So if I had only 5 minutes a day to learn a language, here's the one thing I would do :

5 minutes of flashcards with Anki or a similar app.

During this process, I would :

1. Make sure the cards I review aren't just simple "word -> translation" exercises, but contain audio, example sentences, and so on.

2. Try to be as slow as possible and not answer only in my head, but try to speak and repeat what I hear.

It seems to me like the best return on investment you could get in only 5 minutes a day.