Almost every German politician, most of the police and half the population of Germany speaks English, even though it shouldn't be used for official business. It's no barrier.

Language is also a cultural signifier. English might not be a barrier, but protesting in English certainly communicates that the protesters care not for German culture.

Isolating itself from the world isn't part of German culture either

> Instead of the eyes of their own politicians?

Politicians? It is messages for the voters, and they are leftist politics (so their voters doesn't understand German)