hi! there's room for more input, but you may be interested in these
Bonaparte began to speak and learn French at a relatively young age, before the age of 18 (where it is often considered extremely difficult to learn a language). He was undoubtedly fluent in it, but most contemporary and modern historians who have looked at the man's life acknowledge that it never lost its gruff, Corsican flair. It was described as loud and grating in tone.
In terms of actual problems being caused by French being a learned, and not natural language for Napoleon, the only scholar I've read in recent memory who seems to lend any issue to the fact appears to be Chandler, who remarks that his choice of words in his Correspondence was often ambiguous, or poorly chosen, and may be one (of many far more important) factors of why some of his plans (such as his maneuver near El Arish in Egypt, or the crossing of the St. Bernard pass) courted disaster due to misinterpretation and redistribution by his chief of staff. In the latter example, Napoleon's orders were re-distributed through a middle-body due to constitutional reasons, and they rather stupidly left out two-thirds of the General's plan of operations, leading to a nearly disastrous shift in marching for the intended general (Berthier, commanding the Army of the Reserve).