Isaiah 19:18 says
בַּיּ֣וֹם הַה֡וּא יִהְיוּ֩ חָמֵ֨שׁ עָרִ֜ים בְּאֶ֣רֶץ מִצְרַ֗יִם מְדַבְּרוֹת֙ שְׂפַ֣ת כְּנַ֔עַן
What is שְׂפַ֣ת כְּנַ֔עַן in this context? Why does Yeshayahu identify the language with Canaan instead of directly with Hebrew?
In the entry "Canaanite and Hebrew" in the Encyclopedia of Hebrew Language and Linguistics, we have
In the Bible, Hebrew is once referred to as שְׂפַ֣ת כְּנַ֔עַן śəp̄at kənaʿan ‘the language of Canaan’ (Isa. 19.18), although the more commonly found term is יְהוּדִית yəhūḏīṯ ‘Judahite’ (2 Kgs 18.26, 28; 2 Chron. 32.18; Neh. 13.24; Isa. 36.11).
We find this in the commentaries as well, e.g. Rashi
"מדברות שפת כנען - כישראל שבארץ כנען
the language of Cana'an -- As Israel, which is in the land of Canaan [speaks]
or Rav Kara:
שפת כנען - זהו לשון הקודש.
the language of Cana'an -- This is the Holy Tongue
or Ibn Ezra:
שפת כנען - מזה נלמוד ,כי הכנענים בלשון הקודש היו מדברים.
the language of Cana'an -- From this we learn that the Canaanites used to speak the Holy Tongue
Hebrew was indigenous to Canaan even prior to Avraham arriving there (whose native language was Aramaic) and is therefore associated with Canaan. See Ramban to Bereishis 45:12