"Many Japanese in Japan speak Esperanto."
Translation:Multaj japanoj en Japanio parolas Esperanton.
One of the correct solutions has "en Japanujo" instead of "en Japanio"; what is this construction and why is it correct in one answer but not the other?
Japanujo and Japanio are completely interchangeable. If only one or the other is accepted for a particular sentence, it's because it hasn't been fixed yet.