Funnily enough, AFAIK this is also reflected in the total webpages on the internet. There is an extraordinary amount of German webpages compared to other non-english ones.

EDIT: Third most used language, after English and Spanish.

While that is true, logging into your OS can also be done via 2FA, namely via a smartcard. It’d offer a similar protection to it, if not better.

Honestly, due to how it’s a paid app, I don’t see any viable mass adoption. Possibly great for a professional/corporate setting, but considering that Signal is free and some people already have a hard time leaving WhatsApp, it’d be hard to convince anyone to pay for a messaging app.