I think a lot of people speak excellent English in the Netherlands and he's been there long enough to have learned enough to get by - so I like to think he'll be OK in that department.
Kurto was born in what, 1990, 1991? This means all his primary and high school in Poland was done after the fall of the old regime (1990) which means English and German (or English and French) as compulsory two foreign languages from primary school on. I expect him to communicate in reasonable English well.
And it is not even a new thing: right up till the fall of the Soviet block it was still required to take two foreign languages at high school but one of them had to be Russian (and the other either English, German or French). However, the main problem with fluency in those languages until 1990 was that young people simply had no opportunity to live and work abroad which would promote fluency in the languages they learned. In fact this did not improve until a bit later, probably late nineties (when the permanent work permits were first offered by Germany and then The Netherlands and other EU countries). It changed completely after Poland joined the EU in 2003 and Poles who spoke foreign languages poorly simply struggled getting good jobs.