Well, I assume somebody there speaks Swedish or Danish. Problem solved.
But otherwise, have Sweden hold onto Pomerania for longer. In my opinion you either want to have a strong union of Kalmar type of country, or just have Sweden come out of the Great Northern war victorious and more powerful. Possibly mix in some Scandinavian "colonization" of the region. If Sweden keeps Pomerania for long enough, Bismark (or some analogue) comes along and tries to take it (not sure if they succeed). After that, history is butterflied to the point where prediction gets hard. But if you can get a Pomerania where much of the population speaks Swedish around 1900, chances are that enough will speak Swedish at the present to accomplish your challenge.