Can a layperson help a priest with confession if there is a language barrier and the priest asks for help translating?
Yes, a layperson can translate for a priest and penitent, assuming that there is a language barrier and the penitent wants a translator present. (An interpreter should not be present against the wishes of the penitent.) However, that layperson must know that he is as bound by the sacramental seal of confession as is the priest and may not reveal the contents of the confession under any circumstances whatsoever.
§1 The sacramental seal is inviolable. Accordingly, it is absolutely wrong for a confessor in any way to betray the penitent, for any reason whatsoever, whether by word or in any other fashion.
§2 An interpreter, if there is one, is also obliged to observe this secret, as are all others who in any way whatever have come to a knowledge of sins from a confession (canon 983, Code of Canon Law, emphasis added).