the Title "Heute, Morgen, ..." comes from the firmware. the description how the weather looks like comes from the Mi Fit App. You need to disconnect the Band from original Mi Fit app and uninstall it. now install the
Modified one and connect your Band. with the next syncronisation of the weather it will switch to german (i think maximum 24h). if it does not work try to ask for help at the thread of the modified app, maybe they can help you.