That said, Germany's functional illiteracy rate is estimated to be comparable to the US [0]
It's a similar story with PISA as well - most other countries use it as a dick measuring contest by testing at gifted schools, but the NCES tries to randomize PISA testing in order to actually benchmark subnational performance and identify laggards.
Although I assume illiteracy indirectly implies that you can speak the language in some acceptable form, I would imagine it would be similar to living in another country without knowing the language. You go into the McDonald's click on the pictures of the food you want, press the big green button, then use your plastic card.
And note that very few counties in the US have a score above 276, which is the top of the range described as "Respondents can paraphrase or make low-level inferences."