Interslavic artificial language

It's been contended by certain commentators I've read over the years that linguistically, the Slavic language structurally, even phonemically, closest to Polish would have to be Czech!

Lexically though, is another matter. In the latter case, Ukrainian might be closer:-)

I used to think so too, but Slovak is a lot easier for Poles to understand

Phonemically, you mean, that is, in terms of how the actual pronunciation "sounds"? Lexically aka semantically, I wonder.
Not knowing Slovak, and having only a cursory knowledge of both Czech and Ukrainian, I honestly couldn't judge in any substantive way:-)

Speak Serbian. Central Slavic language. Obed and acept your Sarmatian self. Resist Geak and Italian domination! Resist stan!

Nearly all languages have at least some foreign borrowing, Ratko, even Icelandic!
:-)