C. F. Keil and Franz Delitzsch on Jeremiah 1:7

Jer 1:7

This excuse God holds of no account. As prophet to the nations, Jeremiah was not to make known his own thoughts or human wisdom, but the will and counsel of God which were to be revealed to him. This is signified by the clauses: for to all to whom I send thee, etc. The על belonging to תלך stands for אל, and does not indicate a hostile advance against any one. כל after על is not neuter, but refers to persons, or rather peoples; since to the relative אשׁר in this connection, עליהם is quite a natural completion; cf. Isa 8:12, and Ew. §331, c. Only to those men or peoples is he to go to whom God sends him; and to them he is to declare only what God commands him. And so he needs be in no anxiety on this head, that, as a youth, he has no experience in the matter of speaking.