If you screw a French (one of the two standards: M14 x 1.25) into an Imperial (the other standard: 9/16-20) crank, it will be a bit wobbly, because while the thread pitch is close, the Imperial has about a 2% larger diameter. Threads (both internal and external) should be clean when you try this. You shouldn't be able to screw an Imperial-threaded pedal into a French-threaded crank without brute force. A French-threaded crank can be retapped easily for an Imperial-threaded pedal, but not vice-versa (obviously).
"The great thing about standards is that there are so many to choose from".