When you have at least 3 men who ate a meal together, you can bench using the formula "Rabbosai Nivorech", at which point the other men at the table respond "Yehi Shem..." and so on. However, what happens when you have 3 men at the table, but only two of the men know what to say; one of the men leads the benching, and the second man responds, but the third remains silent because he doesn't know what to say. Are you allowed to continue? I ask because I remember reading a gemara (I think in Berachos) where the whole point of the wording of "Let US bless" is that there is one person who calls to at least two other people to offer the blessing.
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Ba'al HaTania (Shulchan Aruch Harav OC 193:10) states that חרש ואלם if they are not shote can be added to a zimun of 10, if they are not rov but not to zimun of 3 because there you need every one to answer.
However, ביאור הלכה in OC 193 brings a ברכת אברהם saying that one can not be motzi another in birkat hamazon if the listener does not understand. In a case of a zimun it's possible even if the listeners don't understand. If they don't understand Hebrew they probably didn't answer the zimun itself - still they had the zimun and are yotzeh with the benching. Me'iri (Brachos 45) says answering the zimun isn't a bracha but sort of a call to attention, which can explain why they won't have to answer.
(Partially based on Shiur by R' Yair Kahn)
Shulchan Aruch (200:1) [based on Brachos 45b] discuses a case where three people were eating together, and two of them finished and wished to bench, while the third still wished to continue eating. The Halacha is that the third person is obligated to interrupt his eating in order to participate in the zimun, and then afterwards continues eating. In the event that the third refuses to stop his eating and does not answer, Shulchan Aruch rules that although he does not fulfill his own obligation of the zimun, the other two do. From here we see that the zimun is not dependant on all parties answering; as long as there are three people present who hear the zimun, those who do answer fulfill the obligation.
[The Rishonim write that as long as he is present, hears the zimun and "is able to answer", the other two fulfill their obligation. The Mishna Berura (S"K 3) understand that "able to answer" is coming to negate if he left the room, and the Shulchan Aruch Harav explains it to negate if he is deaf/mute but it would seem that not knowing what to say would still fall the category of "able to answer"].