Tongues have two uses, one as a sign and the other as a gift. Tongues are a sign to unbelievers, yet 1 Cor 14:26 tells us to bring tongues to a church service. 1 Cor 14:22 he says that prophecy is not for unbelievers but for believers. Yet in 1 Cor 14:23-25, the unbeliever is repulsed by tongues and converted by prophecy. Interesting twist.
No, the unbeliever would be unconverted by tongues if he did not understand that particular tongue.
That is why a interpreter was necessary.
However, if an unbeliever came in when prophecy (for the purpose of edification) was going on, he would benefit from that.
Also, tongues were to be given in order, so the unbeliever would not consider the church in a state of confusion.
In conclusion, even when tongues was in operation it was considered the least of all the gifts.