From a reader:
I know that the use of Latin is good for large international Masses (and for parish Masses, too…!), but what if, say, a priest and penitent didn’t speak the same language? Could a confession be made in Latin?
Sure! I have done that many times, as a matter of fact.
The Latin Church’s liturgical language is Latin. No! Really!
A flaw in your question: If both the penitent and the confessor know Latin, then they do know the same language.