Q. How do I convert "cuillère" to teaspoon and tablespoon?
Well, are you talking about the French cuillère à soupe or the cuillè à café? Because the first corresponds almost exactly to the US tablespoon and the second almost exactly to the US teaspoon. In fact, while they are theoretically only 1% larger than our spoons, there is so much variation in measuring spoon sizes, that you can absolutely use them one-for-one in your cooking.
Of course, for the British, this means the French tablespoon is one-third smaller than their measuring tablespoon and one-third smaller than their teaspoon. In metric equivalents, a French tablespoon holds 15 ml and the teaspoon 5 ml.
If your recipe just calls for a cuillère, you've got the same problem you would have with a recipe that just called for a "spoonful" of some ingredient figuring out whether to use a tablespoon or teaspoon. In that case, we'd consider the general potency of the ingredient (cayenne vs. rose water) and the relative quantities of other ingredients in the recipe, and make our best guess. What's the worst that could happen?