I want to right screen play, but every time I think about writing, the first question that pops in my head is which one to choose: my native language or English?
Some of the most beautiful prose was written the authors' second languages (Nabokov, Kerouac, Beckett). Writing in different languages gives gives you access to new vocabulary, new phrases, even new ways of structuring stories. Every story you write in one language will will benefit the next one you write in the other.
Of course, writing in a second language is extremely challenging. Your command of the language needs to be as great as a native speaker's. But even native English writers face this problem, when they choose an unfamiliar setting for their story. A UK author setting a story in Boston will need to do just as much research as you to make the local language their own.
The trick is to consume what you write. Are you writing a Polish short story? Then listen to Polish news and read other Polish short stories. Are you writing an English story? Then read English fiction, English news and watch English movies. Ideally set in the place your story is set in. Ideally, ideally, make it non-fiction, so you don't inadvertently copy the lazy habits of other authors.
As for your current predicament, it seems like part of the problem is that you're committing to an entire screenplay. This means you have to get the choice right immediately which is crippling you before you've even written a word. There are many other choices like these ahead, so maybe it's better to lower the commitment. If you commit to writing a couple of one-page shorts, or some 10 minute scenes, you can easily alternate between languages from one to the next. Save the big projects for when you have enough experience to confidently make decisions like these.
At this point in time I think you would be best writing in your native language - you can always translate it to English in a later date, either yourself or with some help.
You definitely shouldn't be letting this question stop you from writing, if nothing else taking your work and translating it from your native language to English is a very good way to work on and improve your English skills! Because (and I'm not saying this to make you feel bad) from your post here your English, while good enough to make yourself understood isn't up to scratch for "serious" writing (yet), and you really don't want any difficulties with the language barrier to distract those reading your work.