Regardless of where it occurs:
Now, is it possible that an interpreter fails to exercise his gift when someone has exercised his gift of tongues? Yes, but that doesn't mean the gift of tongues was false, it means someone else in the church didn't exercise his gift. Paul later says: "someone must interpret."
If no one understands, however, it still doesn't mean your gift was false. It means you should have kept your mouth shut in church and later exercised your gift between you and God. You did something out of order - you didn't necessarily fake it. Paul could have easily said: "if there is not interpreter, then you're faking it."
Yes, you are praying in tongues and you are edified, but it is better to edify your whole person and even better to edify the church. It is not, however, necessarily the case that the person speaking in tongues will be able to interpret:
The mind is unfruitful, but this doesn't mean the person isn't speaking in tongues. Not all the gifts are given in such a way that they are instantly of the highest benefit. We should pray to use them in the best way possible and pray to receive more of them.