"She plans a dinner in one month" - does the sentence imply that she took one month to plan the dinner - or that she plans for the dinner to occur in one month's time? The translation "She plans a dinner in a month's time" was not accepted for me.
The meaning is: she plans for the dinner to occur in one month's time. I assume they wanted a direct translation.
I would accept your translation. I believe in French they say "en un mois" to mean that she takes a month to do the planning, so your answer is a good English disambiguation. (Any French person want to step in?)
Actually, to avoid any ambiguity, I would add a word to the French sentence: "elle programme un dîner POUR dans un mois". If I want to mean that it takes her one month to plan a dinner, I will change the sentence completely: "il lui faut un mois pour programmer un dîner", for example.
Hmmm...I think the translation should be "she is planning a dinner in one month".. not "she plans..."