Thunder is a great mix of functions and UI. The compact mode is really great.

I use Typewise. It’s not open source but there is a offline version. Both of your needed languages are available. Unfortunately the price is very high. But the honeycomb layout is pretty good, once you are used to it.

I use Todoist. Beside the website there are apps for nearly every device and new tasks are simple to add with their natural language support.