The tense of the Japanese sentence is clearly present. 「している」

I think this is wrong.
a three years ... sentence => a three-year ... sentence

Re: "Her husband is serving a three years prison sentence":
(a) the original English was fine. The new version tries too literally to follow the Japanese (直訳).
(b) the new version is bad English. It should be "three-year prison sentence."

Reverted to original version according to the comments.

the tenses are still inconsistent, so the English sentence cannot be correct as is.

I think the tenses are fine. A good idiomatic translation doesn't have to follow the tense of the source precisely. Different languages have different ways of expressing things.

>I think the tenses are fine. A good idiomatic translation doesn't have to follow the tense of the source precisely. Different languages have different ways of expressing things.

Well, the reality remains ONE. Either the husband is still in prison, as suggested by the French translation, either he isn't, as suggested by the German one. One of the 2 has to be wrong, since it is not possible to be out and in prison at the same time...
Which one is right?

Sorry - I was referring to the Japanese & English ones. In both the husband is still in prison. So the French is OK too.