Daddy, I have had to kill you. (line 6)
This line is the first direct address (or apostrophe) that lets us know that this entire poem is written to the speaker's father. Throughout the poem, the speaker returns to this "you," saying "Daddy," just to remind us that, hey, we're talking to somebody here! This entire poem is a communication to the speaker's father, who, as the poem later tells us, was not easy to talk to even when he was alive.
I used to pray to recover you. (line 14)
Prayer is, after all, a way to communicate with God, or with those whom we've lost. Here, the speaker has just started to experiment with ways to communicate with her dead father. We see her struggling with this through the whole poem.
Ach, du. In the German tongue, in the Polish town Scraped flat by the roller Of wars, wars, wars. (lines 15-18)
Now the speaker is giving us a whole new language to work with – German, that she probably picked up from her German father. Throughout the poem, the father is shown as a German – a Nazi, to be specific. So it's appropriate that the speaker gets caught up in the German language. Just like the Polish town gets destroyed by wars, the speaker is destroyed by the German language.