Q. The Rema says that Aramaic and Hebrew are one language. Can I fulfill my obligation to recite Kreias Shema using the Aramaic of Onkelos, even though it is not a spoken language?

A. In question 792 on this forum in regards to counting sefirah in Russian, we wrote; Biur Halocho (beginning of s. 62) in regards to reading Shema in other languages rules that although one complies, it is only if the people of that country know that particular idiom. If they don’t and it is only he and a few others that speak and understand that tongue, he does not comply. Contemporary Poskim debate as to how many people are needed for the Biur Halocho’s requirement. They maintain that as long as it is an established language spoken officially in at least one nation, one would comply, even if only a small minority understands the idiom in the location where he finds himself now. (Megodim Chadoshim – Brochos p. 153, Yabia Omer 5:12:4). Others (Safa Ne’emana 32,) compare it to the proverbial Seventy Languages that were used when the Torah was translated.
Horav Shlomo Miller’s Shlit’a opinion is that Aramaic today does not fulfill the above mentioned requirements and should not be used for reading shema.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a