France and French speaking part of Switzerland use word “Boosté” and I found it in the french dictionnary so I think its no prob to use it. 
edit: I just saw that in fact its indeed written “amélioré par…” and not “boosté par…” in Dawnbringer weapon. So @Rickygervais was right. I must have made a confusion with another troop in the game where its written “boosté par…”. But that said, the problem I wrote about the fact its all yellow troops (allies and enemies) that give the boost to Dawnbringer and not only the yellow allies still need to be corrected in the French description.