The Favourite Movie

We expect "My" or "His" or "Their" or something like that here. Choosing "The" has a strange effect, perhaps making it sound as the movie itself is really the focus and it doesn't matter whose favourite it is - simply that it IS a favourite.

Note: Theis journal is aiming for a formal tone.

No week goes by without me watching a movie.

I don't know if this is ungrammatical, but it feels... loose. More formally, I'd expect this to be closer to "Not a week goes by where I don't watch a movie." Or "in which".

Some people might think movie itself is quite an informal word, but I don't think it matters very much.

There are periods in which I watch movies more often, and other times when I forget about them because I am occupied with work.

It would read better to keep the same noun on each side of the "and other" here. "periods in which ... and other periods when", or just "and others when". But I think this is okay.

Sometimes, after watching a movie, I feel bad forabout the time I could have spent more wisely.

I think "feel bad for" is most natural either as "I feel bad for [some entity that I pity]" or "I feel bad for not spending my time more wisely" - with a gerund, something you did that you regret. Here, "about" works better.

Comma at the start because this is a dependent clause. The sentence is "Sometimes I feel bad ...", and you have embedded "after watching a movie" inside it.

However, they helped me improve my English substantially.

If watching movies no longer helps your English, or you no longer watch movies, this is fine. Otherwise, it's an ongoing process and it's present perfect "they have helped me".

I even watched some of them multiple times, for example, the series of Harry Potter films.

Again, if you wanted to keep the sense of movie-watching being something you do as an ongoing process, "I have even watched".

It has been translated into more than 80 languages, making it popular all around the world.

He and his friends attend a wizarding school, learning everything about witchcraft and wizardry.

Throughout the series, they experience the return of the dark wizard, Lord Voldemort, against whom and hiswhose followers they are trying to fight.

This is an interesting way to try saying this :) It's the kind of thing you say to show off how well you write. I'm not actually sure my suggestion is permissible - it might need to be "against whom and against whose followers".

It may be the best way to say it, I can't think of a formula that preserves all the meaning without being cumbersome. The closest I can get is "They try to fight against Voldemort and his followers, but throughout the series they experience his return." Not exactly the same emphasis, but something you can say without thinking so hard about it.

The wizarding world, in both books and films, gained lots of fans throughout the years, and does not seem to be getting any less popular.

"in both books and films" is again a dependent clause, and the other comma I put in because it just feels like an appropriate place for one. There's probably a rule about it.

Some people like the books more because the story is more detailed.

"Some people like books more" - books *in general*. Some people like books more, while others prefer films or TV. No specific books.