The speaker of "Daddy" is addressing her dead father, who she had problems talking to even when he was alive. Maybe this is because he was a German immigrant and couldn't speak English well, or maybe it was because she was scared of him, but in any case, the German language plays into her difficulties. At the end of the poem, the speaker cuts off communications with her father for good. The speaker's struggle to communicate with her father causes her great suffering, demonstrating the power of language.
It's ironic that though the speaker claims that she's through with her father, she writes an 80-line poem to cut off communication with him.
The speaker expresses her inability to communicate in her father's language, German, in order to show his power over her.