"formed thee in the belly" he's making a reference to Isaiah a bit, using his language to also confirm his calling, the same writing style.
"I sanctified thee" I guess there are some personal pre-mortal requirements to becoming a prophet here.
"unto the nations" this may support the belief that he was chief among the prophets of the time, having such a broad calling. Jeremiah 25:26 "and all the kingdoms of the world" supports this theory.
Jeremiah 1:9
I wonder if touching his lips is some kind of ordinance (or symbolism of one). It involves cleansing sin so it seems likely; it is also tied directly to calling a prophet, maybe the Lord is empowering them as well.
Jeremiah 1:10
Jeremiah's writings have been in the Bible for centuries, and its words have certainly had that effect ("I have this day set thee over the nations and over the kingdoms, to root out, and to pull down, and to destroy, and to throw down, to build, and to plant.")