"The Dutch tourist is running beside the Japanese."
Translation:A holland turista a japánok mellett fut.
The word "Japanese" is not necessarily plural. If the Dutch tourist is running beside the single Japanese tourist, then "japanok" is wrong. The sentence is ambiguous about whether the Japanese tourist is singular or plural.
There is another mistake, in that a correct word order is marked as incorrect. I put "A Holland turista fut a Japanok mellet" and was marked incorrect. Unless I am mistaken, there is nothing in the original sentence to indicate that it is /next to/ the tourists that should be emphasized.