Hibiscus is about the only online banking software usable by Germans under Linux natively. Jameica is the platform on which it is based:

https://www.willuhn.de/products/jameica/
https://www.willuhn.de/products/hibiscus/

Both are open source.

https://www.willuhn.de/products/jameica/releases/current/jameica/jameica-linux64.zip (version: 2.6.4)

https://www.willuhn.de/products/hibiscus/releases/current/hibiscus.zip (version: 2.6.19 - it is normally installed via File>Plugins>Settings>Install new plugin)