I'm not trying to excuse why Bellingham didn't learn German but I think it's partly because Germans usually speak English nowadays and also he never planned to stay in the country.
But like I said before, some just have a hard time learning languages. It makes it even more difficult if you're not interested.
I used Luis Garcia as an example before. He was doing interviews in English after like 6 months.
Also it's a class thing as well, white Brazilians like Lucas, Aurelio, Alisson had some understanding of the English language before they landed in England because they learned in school.
Unrelated but I remember when I travelled to Portugal I was surprised that they all spoke English especially the younger generation. Told a girl in the hotel staff about it and added that I've heard C. Ronaldo trying to speak English and she said that he was a special case because he's from the islands.