The concept of confessing a sin, seams to imply that the person has to subjectively believe they have sinned—otherwise how could they confess? According to Roman Catholic doctrine, does the priest have to understand the nature of the sin, or merely understand that the penitent believes they have sinned? Can a priest hear a confession in a foreign language? I ask this because this seems like the limit case for understanding what the person is saying in the confession booth. What if the sin seems like gibberish to the priest?
There are several aspects to be considered here. The first situation is the general obligation of confessing grave sins. This is addressed in Canon Law, No. 960:
"Individual and integral confession and absolution constitute the only ordinary means by which a member of the faithful conscious of grave sin is reconciled with God and the Church. Only physical or moral impossibility excuses from confession of this type; in such a case reconciliation can be obtained by other means."
The lack of a common language between penitent and confessor would enter into the category of a "physical or moral impossibility" which would excuse either the obligation of confession or its integrity, and allow for reconciliation to be obtained by other means.
In the present case we would be dealing with the confessor making a prudential judgment that the penitent is excused in virtue of a physical and moral impossibility and presuming the latter's sincerity in manifesting those sins confessed in his native language.
Thus in this particular situation the sacrament would be valid.
However, canon law does foresee the possibility of confessing by using an interpreter, although the penitent may not be obliged to do so. To wit: "Canon 990: No one is prohibited from confessing through an interpreter as long as abuses and scandals are avoided and without prejudice to the prescript of can. 983, §2."
Canon 983, §2, requires absolute secrecy on the part of the interpreter analogous to the priest's sacramental seal: "The interpreter, if there is one, and all others who in any way have knowledge of sins from confession are also obliged to observe secrecy."
The violation of the secrecy of confession by an interpreter may be punished by the imposition of a canonical penalty not excluding excommunication (see Canon 1388, §2).
An interpreter need have only a sufficient command of the two languages involved and requires no official certificates of competence.
You can read more here.