Half of everyone in Germany can speak English. It's mandatory to learn in schools. In Berlin that percentage is even higher. So it's no obstacle. And they don't want to limit the visibility of the protest to only Germany.

Germany is one of the more native-language-enforcing countries in Europe. I've been to Slovenia and half of everything is actually written in English, unlike Germany or France where everything's written in the local language.

Because they want to have the eyes of global press on Germany

Instead of the eyes of their own politicians? Are their goals to actually effect change in Germany or to disparage Germany in the eyes of the global press?

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)