John Gill Commentary Isaiah 33:19

"Thou shalt not see the fierce people, a people of a deep speech that thou canst not comprehend, of a strange tongue that thou canst not understand." — Isaiah 33:19 (ASV)

You shall not see a fierce people
A people of a fierce countenance, as in (Daniel 8:23) fierce in their looks, furious in their temper, cruel and bloodthirsty in their practices, confirmed and hardened in their sins, whose consciences are seared as with a red hot iron; a character given of the Papists, (1 Timothy 4:2) these shall be no more seen nor feared:

a people of a deeper speech than you can perceive ;
than the people in common could, having their worship and devotion not in their mother tongue, but in the Latin tongue:

of a stammering tongue, [that you can] not understand :
meaning the same as before, a barbarous language, as everyone is to those who understand it not; so the Syriac and Assyrian languages were to the Jews, (2 Kings 18:26) and so the Roman language to other nations; but now no more to be used in religious worship; nor shall the church of God be any more visited by Turks or Papists, and be in any dread of them more.