I'm not a grandmaster or anything but I've been playing the french for most of my chess career and 2...d5 seems to be the correct response to almost any 2nd move white plays. The game may still transpose to a standard french defense opening later and it still follows general opening principles. ie have some control in the centre early and it still allows further development of your pieces.
For actual grandmaster games its best to refer to the many opening books you can pick up 2nd hand for a reasonable price. In my opinion anything but placing the 2nd central pawn forward is handing a comfortable game to black. Queen's pawn opening players will probably disagree but I had good results against those that didn't.
I actually played 1...e6 to just about any opening e4 d4 c4 in my early chess playing and it nearly always transposed to a french later in the game or a line of the queens gambit or torre attack that i was at least familiar with.
Good luck with the french, it's a cracking opening.
Recently I've been playing 1. e4 e6 as black where white usually plays d4. Beyond that I do know some theory. However, I am unsure of what to play if white develops a knight on either c3 or f6 or what the name of that opening would be?
Does anyone have any input I can't seem to find any high level games like this and my chess engines suggest totally diferent responses so I am guessing it's a mater of personal taste a bit.