I made the french language file, but as minecraft says "translation may not be 100% accurate", because I'm french and I'm not sure to understand all the nicety of seed's names.

I can't send directly the lang file, so here is its content. I hope you could add it to the next version of your mod.
I encourage other people suggesting translation for other languages.

itemGroup.dynamictrees=Dynamic Trees

tile.rootydirt.name=Terre Racineuse
tile.rootysand.name=Sable Racineuse

tile.leaves0.name=Feuilles Dynamiques
tile.leaves1.name=Feuilles Dynamiques

tile.oakbranch.name=Chêne Dynamique
tile.sprucebranch.name=Épicéa Dynamique
tile.birchbranch.name=Bouleau Dynamique
tile.junglebranch.name=Arbre de Jungle Dynamique
tile.darkoakbranch.name=Chêne Noir Dynamique
tile.acaciabranch.name=Acacia Dynamique
tile.cactusbranch.name=Cactus Dynamique

tile.dynamictrees:oakbranch.name=Chêne Dynamique
tile.dynamictrees:sprucebranch.name=Épicéa Dynamique
tile.dynamictrees:birchbranch.name=Bouleau Dynamique
tile.dynamictrees:junglebranch.name=Arbre de Jungle Dynamique
tile.dynamictrees:darkoakbranch.name=Chêne Noir Dynamique
tile.dynamictrees:acaciabranch.name=Acacia Dynamique
tile.dynamictrees:cactusbranch.name=Cactus Dynamique

item.oakseed.name=Gland de Chêne
item.spruceseed.name=Pomme d'Épicéa
item.birchseed.name=Graine de Bouleau
item.jungleseed.name=Cosse d'arbre de Jungle
item.darkoakseed.name=Gland de Chêne Noir
item.acaciaseed.name=Graine d'Acacia
item.appleseed.name=Pépin de Pomme
item.cactusseed.name=Graine de Cactus

item.dendropotion.biochar.name=Potion Biochar
item.dendropotion.depletion.name=Potion d'Épuisement
item.dendropotion.defoliance.name=Potion de Défoliance
item.dendropotion.burgeoning.name=Potion de Bourgeonnement
item.dendropotion.fertility.name=Potion de Fertilité
item.dendropotion.persistance.name=Potion de Persistance
item.dendropotion.transform.name=Potion de Transformation

item.dirtbucket.name=Saut de Terre
item.staff.name=Bâton Forestier
tile.dendrocoil.name=Dendrocoil
tile.bonsaipot.name=Pot pour Bonzaï

Thank you. Translations are always welcome :) I'll add this before the next release.