Egyptian language
Egypt , allover its history, didn't change its language except one time only
from Egyptian to Arabic (or the modern colloquial language ).
The hieroglyphic , hieratic , demotic & Coptic are different scripts to one
language only, which is the Egyptian language.
of course there had been some changes & evolutions along its long history,
but the language as a whole remained the same.
"something like the difference between English at the time of Shakespeare " , for example,
and the modern English. it is one language : English, although the changes which occurred.
Another example, about different scripts to one language;
is the Turkish language, which was written in Arabic letters for hundred years, then they
wrote it in Latin letters at the time of Kamal Ataturk in the
20th century. the didn't invent a new language or change it, they've just
written their spoken Turkish language, in Latin letters, instead of the Arabic one.
So Coptic is the latest script of the Egyptian language,
actually it is wrong , or a common mistake to call 'Coptic', a language, it is
the Egyptian language, written in Coptic letters.
so , instead of saying this long phrase, " the Egyptian language written in Coptic letters",
most of the people say it in short : the Coptic language; which is not correct.