Well, I passed through Liegnitz... not recently but in 2001 on the train from Krakow to Berlin, direct, only saw the parts of the station. The train station sign shows only "Legnica" (famous battle site too). What I have seen in towns/cities formerly German, visited such as in West Prussia, Gdansk, Malbork, Torun, Chelmno on my three trips there, the street names are all in Polish, as are signs pointing to cultural and historical sites, which include English, sometimes German too. . But regarding these signs you'll see Polish and English. You speak German..great. In Gdansk at the Pension the two women who ran it didn't speak English or refused to do, so among the 3 languages (Polish, Russian, German) available to them, I used German, which came in very handy due to a discrepancy on the final bill,,,another story, but they saw they were wrong. In Chelmno at a restaurant for lunch I used German but that waitress spoke English just as well as German. At the small hotel in Torun and restaurant I used English. In the hotels expect the staff to be professional speaking to you in very good English. Some speak also German, very accommodating. If you go by train and the station clerk is limited in English, they'll get someone better to answer your questions. My last trip was in 2005 (Torun)...not too recent , I know but I've witnessed/experienced what is described above. In restaurants expect to see them in Polish, German, English, etc. although I did see a menu in a border town the other side of Küstrin an der Oder only in Polish and German.
"Is there anything that can be seen from pre-WW2?" The answer to that, historically, is no, unless you mean churches stemming from the Middles Ages that have been restored, refurbished, etc, but the historical German names are gone. What I saw was mainly in the lower Vistula valley, which is not where you want to go. I looked for the same from a historical perspective. Only some farms/houses I saw from the bus going from Torun to Chelmno (used to be called Kulm in Kulmerland/West Prussia were most likely German in terms of the colour paint and building style.
Going by train Berlin to "Legnica" you'll probably have to change in Wroclaw or Cottbus (Ger). Expect it to be a long haul.