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The language can now be switched inside the app
Settings … → Language: System language · Deutsch · English, with a restart button next to it — the bundle resolves its language once at launch, so a fresh process is needed.
macOS can already set a language per app, under General → Language & Region → Applications. The app writes that very same preference; it just puts the switch somewhere people will actually find it.
The language names stay in their own language, so anyone who lands on English by accident can find the way back.
Also fixed
The picker showed "Deutsch" when nothing had been chosen. UserDefaults.standard reads through to the global domain for AppleLanguages, where the system list always lives — the app now asks its own domain only.
macOS 14 or later, Apple Silicon. Not notarized — right-click → Open on first launch.