R. Yehiel Mikhel Epstein, Arukh Ha-Shulhan, Orah Hayim 185:1-3:
The grace after meals is recited in any language, as it says "And you shall bless the Lord your God" (Deut. 8:10) -- in any language that you bless (Sotah 33a). It seems to me that this is only when one does not understand Hebrew, and so it seems from the Jerusalem Talmud...
In the prior generation, a wicked group outside of our country arose to pray and recite the grace after meals in the vernacular and their source is the Mishnah in Sotah "These are recited in all languages..." The great Torah scholars of that generation refuted their claims. I say that this wicked group did not understand at all the words of the sages of the Talmud. That Mishnah lists many things that are recited in any language and the Gemara adduces for each one reasons why it may be recited in any language. The Gemara also lists many things that are recited specifically in Hebrew and brings reasons for each one why this is so. However, it is unclear because if logic dictates that everything should be said in any language, why does the Gemara give a specific reason for each one that may be recited in any language? And if logic dictates that everything should be said in Hebrew, why does the Gemara give reasons for each one that must be recited in Hebrew?
The matter is as follows: Since the Torah was given in Hebrew and Hebrew is the holy language through which the world was created, it is certainly an obligation that anything related to Torah require Hebrew for one who understands it. And we would not permit another language for one who does not understand Hebrew if not for specific verses that permit it... Therefore, the use of Hebrew is not an absolute requirement and one who understands Hebrew and improperly prays or recites grace in another language fulfills his biblical requirement even though he committed a sin. Because it does not make sense to say that one who does not understand Hebrew can use another language ab inition while one who understands Hebrew may not even ex post facto. Therefore, we need specific reason to permit one who understands* Hebrew to use another language ab initio. And we need specific reason to require Hebrew even post facto.
It is clear that it is absolutely forbidden for us to pray and recite grace in the vernacular.
* The text reads one who does not understand Hebrew but I believe it to be a mistake and should read: one who understands Hebrew.