The sources are very controversial. Is there any agreement if I can use полячка, or is it полька?
It's obsolete and currently considered the incorrect form with "полька" being the only valid form. In the third edition of Dahl's dictionary, the editor, famous linguist Иван Александрович Бодуэн де Куртенэ stated that:
Полячка ж., презрит.[ельное], вм[есто] полька.
What is not immediately obvious from his name, Бодуэн де Куртенэ was also of Polish roots and was a strong proponent of Polish (at least) cultural autonomy. I'm mentioning this only to assume that he did know how exactly Polish community felt about that word. By the way, @YellowSky’s comment is absolutely correct, it indeed sounds very offensive to a Polish speaker.
So, stick with "полька", that would be my advise. In that aspect situation is somewhat close to what's happening with word "негр" right now. For millions of native speakers (me included) it was a neutral term pretty much like "шатен" or, don't know "блондин" with honestly zero racist connotation. Because Russian does not exist in vacuum, because it's evolving and there's still such thing as international cultural context, by many people it gradually came to be perceived as something inappropriate. It's quite a flammable topic, but I'm only describing here current state of things and ongoing tendencies.
However I also need to stress out that the situation is not like with, say, "жид", which transformed from relatively neutral term to an undoubtedly derogatory term.
"Полячка" is still used by many interchangeably with "полька" and many of its users even have no idea that one version is more preferable than the other. I guess because it sounds like, say, "москвичка".