Normally the right leg comes first, but occasionally the left leg can come out first and the right leg will follow. However, it is possible there could be a developmental issue with the right leg and it will not pop, or if it does, it will be deformed. In any case, there's nothing you can do and there's no way to pop the leg out yourself safely. The proper emergence of the last leg actually triggers hormonal changes that cause the frog to properly metamorphose, so you don't want to interfere.