Of a strange speech and of a hard language, etc.; literally, as in margin, both of Authorized Version and Revised Version, to a people deep of lip and heavy of tongue; i.e. to a barbarous people outside the covenant, Chaldeans, Assyrians, Scythians: not speaking the familiar sacred speech of Israel (compare the "stammering lips and another tongue" of Isaiah 28:11; Isaiah 33:19).
The thought implied is that Ezekiel's mission, as to "the lost sheep of the house of Israel" (Matthew 15:24), was outwardly easier than if he had been sent to the heathen. With Israel there was at least the medium of a speech common both to the prophet and his hearers.
In verse 6 the thought is enlarged by the use of "many peoples."