Translation:It is not cold water in the bathtub but warm .
Why is the word "hot" used to translate "meleg"?
I learned years ago, that "meleg" means "warm" and "hot" is "forro" in hungarian.
I think there is some overlap, it is not a well-defined line between the two. But in general, the distinction is correct.
Is that overlap in hungarian like in english?
Would an hungarian let a child take a bath "forró vízben" or only "meleg vízben"?
Sure, if the child doesn't mind. One person's hot/forró is another's warm/meleg, and vice versa. Basically, "too warm" is "hot" in everyday usage. And then, when it is really high temperature, one that can cause injury, that is "hot".
When it is really not that warm, more like lukewarm, the Hungarian word is "langyos". Or you can also say "kézmeleg" (like body-temperature), especially for bath water for a child.
Should it not be: There is no cold water in the bathtub, but (rather) warm. ? If I had to translate the English Duo sentence into Hungarian, I would write: Ez a viz a kádban nem hideg, hanem meleg. What do you think about it?
This sentence is just horrible in English, but when I translated it as "the water in the tub is not cold but warm " Duo marked it as a mistake. I cannot see why my translation was not allowed.
In general there seems to be no difference, but there is a little difference between those two english sentences.
The English version of Duolingo would be correct as well by using the formulation "There is..." . Such sentences require in Hungarian the verb "van" because of focussing the existence of something.
Your sentence focusses on the quality of the water. That kind of "it is"-verb is not expressed in Hungarian. They would say: "A kádban a víz nem hideg hanem meleg." / "A víz a kádban nem hideg hanem meleg."
If you understand German:
"A kádban nem hideg víz van, hanem meleg." = In der Wanne ist kein kaltes Wasser sondern warmes. / In der Wanne ist kein kaltes sondern warmes Wasser.
"A kádban a víz nem hideg hanem meleg." = Das Wasser in der Wanne ist nicht kalt sondern warm.