Why flats rantings here require commitment contract for like 9 months or a year at least?
The owners don't want to have the flat empty several times a year with no income.
In other countries you can rent for a day, week, month, whatever as long as you pay for each period.
You can in Poland, though that type of flat has a higher rental cost.
I mean I can't guarantee that I'll stay for the period I'm signing a lease for, what happens if I rented a flat for a year, and wanted to leave after like 2 or 3 months for any reason?
No problem usually. Most rental contracts require that you give a month's notice.
If you aren't planning on staying too long, you should still say that you're looking to stay for a year at least.