A different kettle of Fish

Something different entirely/Completely unrelated to something previously mentioned.

Originates from another idiom which has since gone out of fashion. “Kettle of fish” meaning a mess, perhaps due to the bones head and tail left after eating a fish. This metaphor later evolved to mean “state of affairs”, thus a different kettle of fish. Ex: I'd driven an automatic for years but learning to handle a car with gears was another kettle of fish altogether. Spanish: Eso es harina de otro costal

 German: Das steht auf einem anderen Blatt