"That honest man speaks also French."
Translation:Tuo rehellinen mies puhuu myös ranskaa.
Agreed, and it's what I tried. Should definitely be accepted.
I think the reason it isn't is that the word-for-word translation of "That honest man also speaks French" would be "Tuo rehellinen mies myös puhuu ranskaa". It's clumsy (just like the "also French" sentence) and would likely be understood as the honest man not being the only one who speaks French.
Whereas the English sentence in the example can only mean that the honest man speaks French and at least one other language, which is reflected in the Finnish translation.