Poland needs to have remained a great power. That would give it a prestige and usefulness. So it would likely require a pre-1900 POD.
Spanish, French, and German are the de facto foreign languages taught among English speakers. That is because Spain and France were historical great powers with large empires with lots of inhabitants. France in particular was cultural dominant and had been for a long time. German is taught because it became a great power in the 1800s, and Germany dominated academics, engineering, chemicals, and science before WWII as well as the German economic might once it was unified. Earlier than that, German law and craftsmanship had been prized and was atrractive for much of eastern Europe int he Middle Ages, Magdeburg and Lubeck law was applied, and everyone asked for German settlers to create towns.
For Polish to be a language that is offered, Poland needs be equally useful so that people see the benefit of learning Polish. Since Poland never had a colonial empire, it needs to be very successful in Europe and attain some kind of dominance in critical technical fields. This is very hard to see happening post 1900. Even if Poland does better IOTL post 1920 (say Pilsudski is able to united much of Ukraine and Belarus into a single confederation and becomes the dominant power in eastern Europe), it is extremely poor. Its economy is very undeveloped. Even if it does well, Poland is unlikely to develop the prestige and technical excellence until the very late twentieth century at earliest.
So I think the POD needs to be back in time so that the Polish-Lithuanian Commonwealth is never partitioned - perhaps even back to prevent the Khmelnitsky Uprising and the Deluge. If Poland is the dominant power in Eastern Europe, and its universities and high culture flourish, then Polish might be attractive enough so that the language is something other people want to know.