Complex sentence: The sound "kero" in Japanese, as in the title, is the "ribbit" or croaking of frogs, which, in the series, is used frequently by the frogs and also by Mimori, as she is the princess of the kind.
Split into simpler sentences: The sound "kero" in Japanese (as in the title) is the "ribbit" or croaking of frogs, which, in the series, is used frequently by the frogs. It is also by Mimori, as she is the princess of the kind.