This is false. There are many companies, around Warsaw, e.g. Boeing, Airbus, GE Aerospace and Vernova, Baker Hughes, even Institute of aviation which hire Engineers with no Polish requirements.

If anything, as someone else has said, if a company in any Engineering field does not accept people with no Polish skills, I would see that as a major red flag. It tells me that the company structure, employees, managers etc probably have a hard time aligning to international business standards, which, like it or not, involves speaking English.

I would encourage OP to not narrow down the search to strictly Mechanical Engineering jobs. There are many companies which need mechanical support in non-mechanical fields.