# of strange speech or a difficult language

"who speak a strange or difficult language"

# not to many peoples of strange speech

"I am not sending you to a mighty nation whose people speak a strange language"

# if I sent you to them, they would have listened to you

This is a situation that could have happened, but Yahweh did not send Ezekiel to people who didn't understand his language.

# if I sent you to them

The word "them" refers to a mighty nation other than Israel.