It's not good on mobile. I tried making it language free, and relatively easy to figure out if you explore the buttons and see what happens. The top middle number is your problem. Your task is to click on the solution. If the solution is wrong, the screen flashes red. The bottom shows statistics of how much time you spend on each problem. The top left and right numbers lets you decide what factors are chosen in the problem. The sudoku board lets you toggle showing you relevant solutions based on chosen factors.