Film zaczyna się za pięć minut.

Word by word:

• Film = film / movie
• zaczyna = begins / starts
• się = a particle used with some verbs; here it is part of zaczynać się = to begin / to start
• za = in when talking about time from now
• pięć = five
• minut = minutes

So the whole sentence means The movie starts in five minutes.

Because in Polish, the natural verb here is zaczynać się or zacząć się, which means to begin / to start.

Compare:

• Zaczynam film. = I am starting the movie.
  Here someone actively starts something.
• Film zaczyna się. = The movie is starting / begins.
  Here the movie itself is beginning.

So się is not really translated word for word into English here; it is just part of how Polish expresses to begin in this kind of sentence.

The dictionary form is zaczynać się.

• zaczynać się = imperfective, to be starting / to start in a general or ongoing sense
• zacząć się = perfective, to start / to begin as a completed event

In this sentence, zaczyna się is present tense and means is starting / starts.

In time expressions, za often means in, as in after this amount of time from now.

Examples:

• za minutę = in a minute
• za godzinę = in an hour
• za tydzień = in a week

So za pięć minut means in five minutes.

Because after numbers 5 and higher, Polish usually uses the genitive plural form of the noun.

So:

• jedna minuta = one minute
• dwie / trzy / cztery minuty = two / three / four minutes
• pięć minut = five minutes
• sześć minut = six minutes

That is why you get minut after pięć.

Minut is the genitive plural of minuta.

This happens because the numeral pięć requires the following noun to be in the genitive plural:

• pięć minut
• dziesięć godzin
• siedem dni

So even though the whole phrase za pięć minut means in five minutes, the noun itself appears as genitive plural because of the number.

No, Polish word order is fairly flexible.

The neutral version is:

• Film zaczyna się za pięć minut.

But you can also say:

• Za pięć minut film zaczyna się.
• Za pięć minut zaczyna się film.

These all mean basically the same thing, but the emphasis changes a little:

• starting with Film focuses on the movie
• starting with Za pięć minut emphasizes the time

The original sentence sounds very natural and neutral.

Yes, absolutely.

• Film zaczyna się za pięć minut = The movie starts/is starting in five minutes
• Film zacznie się za pięć minut = The movie will start in five minutes

Both are natural.
The first uses the present tense of an imperfective verb, which Polish often uses for scheduled future events.
The second uses a perfective future form and sounds a bit more explicitly future.

Because the subject is film, which is singular.

• film = singular
• zaczyna się = singular verb form

If the subject were plural, the verb would also be plural:

• Filmy zaczynają się za pięć minut. = The movies start in five minutes.

Yes. In this sentence, film means a movie / a film as a specific event that is about to begin.

For example, you might hear this at a cinema, at home, or before a TV broadcast.

Yes. In everyday writing, za 5 minut is completely normal.

• za pięć minut = fully written out
• za 5 minut = same meaning, more casual/practical

In speech, of course, you would say pięć.

A rough pronunciation guide is:

Film za-CHI-na shyeh za pyench MEE-noot

A few important points:

• cz sounds like English ch in church
• się sounds roughly like shyeh
• ę in pięć is a nasal vowel; learners often approximate it, and that is fine at first
• ó is pronounced like u, though there is no ó in this sentence
• stress in Polish is usually on the second-to-last syllable:
  • za-CZY-na
  • MI-nut

Yes, very naturally.

It is a normal everyday sentence for a scheduled event that is about to begin. A Polish speaker could say it in many situations, for example:

• at the cinema
• before a class or presentation
• when turning on a movie at home

It sounds completely idiomatic.