Small Polish companies will wait to get the money in from the job before they payout to the service provider.
My wife and I have been working regularly with half a dozen translation agencies, as service providers, for over ten years, and we have individual agreements with each of them concerning payment deadlines. We usually receive payment 15 days to a month after invoicing the agencies. It is important to agree terms before taking on jobs. We've had only a few problems over the years, mainly with smaller firms being shafted by big clients and not being able to meet payment obligations. The OP will, I hope, learn from this experience.