The Nubian language can not be used to decipher Meroitic because the Nubians were not members of the Meroitic confederation. They were not Kushites. In fact we only see Nubians in Meroitic art as conquered people.
The textual evidence from the Romans suggest that the Noba/Nubians entered Nubia long after the Kushites had founded the Napatan and Meroitic civilizations, so eventhough they live in Nubia today, they are not representative of Kushite people. The Kushites were often in conflict with the Nubians.
In addition, Egyptian documents make it clear that the Blymmes also entered the area after the founding of the Napatan and Meroitic civilization, so even if some people claim that the Beja=Blymmes this is conjecture. Consequently, even if Beja= Blymmes, they do not represent the Kushite people who founded the Napata and Meroe civilizations, because both the Noba and Blymmes entered Kush after its founding.