Oszczędzam dziewięćdziesiąt złotych.

Because Polish usually drops subject pronouns when the verb already shows who is doing the action.

Oszczędzam already means I save / I am saving, so ja is not necessary.

You can add ja for emphasis or contrast:

• Ja oszczędzam dziewięćdziesiąt złotych. = I am saving ninety zlotys.

It is the 1st person singular present tense form of the verb oszczędzać.

So:

• oszczędzać = to save, to economize
• oszczędzam = I save / I am saving

The ending -am is what tells you it is I.

It can mean either one.

Polish present tense often covers both:

• a habitual meaning: I save ninety zlotys
• an ongoing meaning: I am saving ninety zlotys

The exact meaning depends on context.

Also, oszczędzać is an imperfective verb, so it naturally describes an ongoing, repeated, or general action.

Because after numbers 5 and higher, Polish uses the noun in the genitive plural.

That is why the currency word changes like this:

• 1 złoty
• 2, 3, 4 złote
• 5+ złotych

So:

• dziewięćdziesiąt złotych = ninety zlotys

This pattern is very important with Polish numbers.

The noun złotych is in the genitive plural.

A helpful way to think about it is:

• the whole phrase dziewięćdziesiąt złotych is the amount being saved
• but after numerals like dziewięćdziesiąt, the noun itself appears in genitive plural

So even though the phrase functions as the object of the verb, the noun form you use is still złotych.

The dictionary form is złoty.

So:

• złoty = zloty, the Polish currency unit
• złotych = a declined form used here after dziewięćdziesiąt

You will often see the abbreviation:

• 90 zł

But when spoken out fully, it is:

• dziewięćdziesiąt złotych

Yes.

Depending on context, oszczędzać can mean:

• to save money
• to economize
• to spare
• to save on something

Examples:

• Oszczędzam pieniądze. = I save money.
• Oszczędzam czas. = I save time.
• Oszczędzam prąd. = I save electricity / conserve electricity.

In your sentence, because there is a money amount, it clearly means saving money.

Yes, Polish word order is more flexible than English.

The sentence Oszczędzam dziewięćdziesiąt złotych is perfectly natural and neutral.

Other orders are possible, but they change the emphasis:

• Dziewięćdziesiąt złotych oszczędzam. = It is ninety zlotys that I am saving.

So the original sentence is a very normal way to say it.

This sentence contains ę and ą, which are nasal vowels in Polish.

For a learner, a practical approximation is:

• ę in oszczędzam often sounds somewhat like en/em before certain consonants
• ą in dziewięćdziesiąt often sounds somewhat like on/om before consonants

So you do not always pronounce them like pure nasal vowels in every environment. Their real sound changes a bit depending on what follows.

That is normal in Polish pronunciation.

A common perfective counterpart is zaoszczędzić.

So:

• Oszczędzam dziewięćdziesiąt złotych. = I save / I am saving ninety zlotys.
  This is imperfective.
• Zaoszczędzę dziewięćdziesiąt złotych. = I will save / I will manage to save ninety zlotys.
  This is perfective.

The imperfective version focuses on the process or habit. The perfective version focuses more on reaching the result.

Yes. It is very useful to memorize Polish money expressions as number patterns.

A good mini-pattern is:

• jeden złoty
• dwa złote
• trzy złote
• cztery złote
• pięć złotych
• dziesięć złotych
• dziewięćdziesiąt złotych

If you learn this pattern early, it becomes much easier to talk about prices and amounts in Polish.