Halacha Orach Chaim 185

Birchas Hamazon can be said in any language; however, it is best to recite it in Lashon Hakodesh. The Sefer Hachinuch writes that one who is meticulous with Birchas Hamazon will be supported respectfully throughout his lifetime.

Biyur Halacha by Hilchos Tefilla writes that a G-d fearing person may Daven in another language if it will increase his concentration. Other than such a person, one that is capable of reciting the prayers in Lashon Hakodesh should do so.

Shulchan Aruch Harav writes that if one does not concentrate on the words he is saying, he will not fulfill the obligation of Benching. The Aruch Hashulchan comments, that it cannot be the Mitzva is not fulfilled. Unfortunately, a high percentage of people do not concentrate appropriately. It cannot be that most people are neglecting the fulfillment of Birchas Hamazon. With this reasoning he writes that it must be that one can fulfill a basic Mitzva even without concentrating properly. Sefer Chasidim brings a story that describes the severity of not Benching appropriately.

The Mechaber writes that one is obligated to hear the words he is saying.
Reb Shlomo Zalman rules that one can fulfill the Mitzva, as long as they words are recited, even though no sound actually comes from his throat.