First, mortal sins require a true act of repentance, a supernatural act of repentance, which is expressed through confession. At this moment, God purifies the soul from sin, and this is the essential act in the sacrament of confession.
In this case, when the priest understands what is being said during the confession, we can assume that he grasps the essential part of the confession, and we don't need to worry about its validity, as long as we sincerely confess our sins. However, we cannot confess to a priest who completely does not understand anything. But in this situation, we can assume that the priest at least understands the basics and that one should not confess to someone who does not understand the language.