Mine is rusty as all hell. Polish was the first language I learned (growing up in a Polish-only speaking household in Chicago), and then when I started socializing with the neighborhood kids and watching TV, that’s when my English came in. Or perhaps you can say I learned them concurrently.
Anyhow, I primarily know its grammar by ear, and this would be the biggest hurdle in teaching it for me. For example, in the phrase above for two beers, it would correctly be: Dwa piwa proszę or dwa razy piwo, proszę. But I had to double check, even though that’s how I would say it, because I don’t know the formal rules, just what sounds right. The grammar is a real pain in the ass. Correctly declining numbers are especially tricky. Polish has seventeen forms for the number two, depending on its grammatical function in the sentence and the word it is modifying (if it is being used as an adjective.) Here’s how “One/Two/Five windows” would decline as the subject of a sentence: jedno okno, dwa okna, pięć okien. What a pain in the ass.
The good news is, getting the case system wrong isn’t going to really hamper your ability to be understood. It may hamper your comprehension a bit if you don’t know all the forms, especially for shorter words where you have less data to go on and a grammatical change in form is more likely to interfere with your recognition of the unadorned (nominative singular) form.