French Defence

When playing the French defence, how would I respond to irregular moves such as 2. d3 rather than 2. d4?

This is the King's Indian Attack vs the French, something Bobby Fischer used to roll out now and then. Instead of re-posting material that has already been posted elsewhere, I'll just leave you this link where there are some interesting ideas you can try: https://www.chess.com/forum/view/chess-openings/the-french-vs-kia-is-my-current-problem

Thank you very much!

The answer in general terms is to try to play the same moves you'd play in a normal French, ie, d5, c5, Nc6, Nf6 (or e7) and the dark-square bishop to b4 or e7 as appropriate. Look for pressure on the White centre and the chance to either develop or exchange the light-square bishop.