I'm looking for a dataset containing crossword grids along with their clues and their solution, preferably in English or French. If possible crossword grids would be rated on some difficulty scale and dated.
Anyway, his data source was XWord Info, which shares all the NYTime Daily Crossword Puzzles. You can see an example (most recent) of JSON format:
Details here. With a snippet of code you can search over all valid calendar dates, then download and parse.
There is also the XPF format which is designed for crossword puzzles in order to fit unusual features.
For NYTimes puzzles, the difficulty generally increases from Monday to Sunday.