Qalculate!, the calculator I use every time I need to do a calculation, especially if it involves units or currency conversion. Does everything I’ve ever needed out of an everyday calculator (even symbolic calculation and exact results), while keeping the usual simple calculator interface.

I’m aware of LWT and LUTE with the same concept. Neither comes with predefined languages or texts, so they should work for any language as long as you have some texts you want to read.