Why is this one verse in the book of jeremiah written in aramaic when the rest is in hebrew?

Probably it was added in during the second temple period 539 B.C. - 70 AD. when aramaic was the common language of the Israel area. There came a point when it replaced Hebrew as a written language, I don't recall when that was, but it makes sense that that was when the aramaic portions of the bible where written. Hebrew scribes revived Hebrew as a written language though it didn't become common again until modern Israel. Good question, a study of this could nail down some dates for biblical authorship.

It seems that it is this Official Aramaic that is found in the writings of Ezra, Jeremiah, and Daniel. The Scriptures also give evidence of the fact that Aramaic was a lingua franca of those ancient times. Thus, in the eighth century B.C.E., appointed spokesmen for King Hezekiah of Judah appealed to Assyrian King Sennacherib’s representative Rabshakeh, saying: “Speak, please, to your servants in the Syrian [Aramaean, and hence, Aramaic] language, for we are listening; and do not speak to us in the Jews’ language in the ears of the people that are on the wall.” (Isa 36:11; 2Ki 18:26) The officials of Judah understood Aramaic, or Syrian, but evidently it was not understood by the common people among the Hebrews at that time in Jerusalem.

A number of years after the Jews returned from Babylonian exile Ezra the priest read the book of the Law to Jews assembled in Jerusalem, and various Levites explained it to the people, Nehemiah 8:8 stating: “They continued reading aloud from the book, from the law of the true God, it being expounded, and there being a putting of meaning into it; and they continued giving understanding in the reading.” This expounding or interpreting may have involved paraphrasing the Hebrew text into Aramaic, Aramaic possibly having been adopted by the Hebrews when in Babylon. The expounding also, no doubt, involved exposition so that the Jews, even if understanding the Hebrew, would comprehend the deep significance of what was being read.

The Tanakh (what Christians call the Old Testament) was written in Hebrew because that was the language of its keepers. Then Christianity and Islam later came along, based on that same religion -- so it's not exactly surprising that these events occurred in the same area. It would be really strange if Christianity arose in, say, Chile.

jeremiah 10:11 can be found in psalm 96:5

I didn't know that. Neat fact.. I have no idea