Halacha Orach Chaim 187

Although one is required to recite Birchas Hamazon in the language established by the Chachomim, if someone mentions the necessary concepts of Birchas Hamazon in place of the Brachos the Rabbis instituted, they fulfill their obligation. In the first Bracha one most mention the fact that Hashem sustains us. In place of the second Bracha we must make sure to mention the inheritance of the land, Bris Milla and Torah. In the third Bracha one must mention the Davidic Dynasty.

It is interesting to note that Reb Moshe Feinstein in Cheleck 1, 74 rules that one who does not have a Siddur and does not know it from memory, should recite a Borei Nefashos in place of Birchas Hamazon, if he has no other solution. There is reason to say that Birchas Hamazon should be repeated if a Siddur is found at a time that Birchas Hamazon can still be said… before digestion. The reason for this is because Boreh Nefoshos should not even be recited in place of Al Hamichya in extenuating circumstances, it certainly should not be said in place of Birchas Hamazon.

There is a disagreement among the commentaries if one is required to Bench if Al Hamichya is recited in place Benching by mistake. Aruch Hashulchan says it works, and Chaye Adam says it does not work.