ΔE 17.5, loose: different enough that you should pick on purpose. Ardennes Green is a little darker, warmer than Medieval Forest.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ardennes Green is a little darker, warmer than Medieval Forest.
The same difference applies the other way round: Medieval Forest is a little lighter, cooler.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Medieval Forest on the colour wheel See Ardennes Green on the colour wheel
All matches: Medieval Forest equivalents · Ardennes Green equivalents