שנים שאכלו אע"פ שבברכת המוציא פוטר א' את חבירו מצוה לחלק שיברך כל אחד ברכ' המזון לעצמו בד"א כשהיו שניה' יודעים לברך ברכת המזון אבל אם אחד יודע והשני אינו יודע מברך היודע ויוצא השני אם מבין לשון הקודש אלא שאינו יודע לברך וצריך לכוון מלה במלה לכל מה שיאמר: הגה וצריך המברך שיכוין להוציאו [מרדכי ר"פ שלשה שאכלו וב"י בשם סמ"ג]: אבל אם אינו מבין אינו יוצא בשמיעה אבל שלשה שאכלו אינ' רשאים ליחלק ...
If two people eat together, even though with regard to the Motzi one of them can recite for the other, they should split up to recite Birkat Hamazon each on their own. When does this apply? When both of them know how to recite Birkat Hamazon. But if one knows and one doesn't know, the one who knows recites and the other fulfills their obligation [by saying "amen"] if they know the Holy Tongue, but don't know how to recite the blessing. And they should pay attention word by word to what is said. ... But if they don't understand, they don't fulfill their obligation by listening. But three who ate together are not permitted to split up....
It's established and clear halachah that Birkat Hamazon may be said in any language that one understands. As we'll see, there's some disagreement about the relative priorities of blessing in Hebrew versus understanding what you're saying. Here, Yosef Caro, the "Composer - M'haber" of the Shulchan Arukh, implies that in a twosome, one who doesn't understand Hebrew must say Birkat Hamazon in a language they understand. But in a threesome, which comprises an official community of 'blessers,' they must participate with the community regardless of their understanding of the blessing.
Turei Zahav (Ta"Z) Magen Avraham
(David ha-Levi Segal, c. 1586 – 1667, Poland)
Comment #2 on the above:
In the Tur, he [Jacob ben Asher, the author of the Tur (13th-14th cent.), whom Caro is mostly quoting] concluded "and therefore women do not fulfill their obligation through listening if they don't understand." And the Beit Yosef [Caro's commentary on the Tur] cites Rashi to the effect that women do fulfill their obligation through listening even if they don't understand. And even though that's not our conclusion, nonetheless with regard to our women, who don't understand the Holy Tongue and most of them don't say Birkat Hamazon on their own, anyway, they could depend on Rashi and, even as the normative practice, fulfill their obligation through what they hear. Anyway, that's much preferable to them not saying the blessing at all. So it seems in my humble opinion.
Caro, in the Bet Yosef, doesn't tell us where Rashi makes the claim that women can fulfill their obligation to say Birkat Hamazon by saying "amen" to a Hebrew rendition they don't understand. It doesn't seem to appear in any of our currently-known Rashi texts. But we can trust that he had some source in which it did appear. Many later commentators depend on this Rashi, as the Ta"z does, to find a way to include women in Birkat Hamazon, even if they couldn't imagine, in their social/ideological circumstances, educating women enough that they could say it and understand it on their own. Other traditional commentators can imagine that, and indicate that educating women would be the better solution.