Without a little research, we wouldn’t even know to ask the question “Why is the book of Daniel written in two languages?” But now that you have heard that Daniel features two languages, let’s talk for a moment about the “why?”.
Like many questions about the Bible, we can only do some light speculation because Daniel doesn’t say, “Please note that I’m writing in both Hebrew and Aramaic for XYZ reason.”
Hebrew and Aramaic are very closely related. They are both called Semitic languages. The section of Daniel from chapter 2 verse 4 through chapter 7 is written in Aramaic while the rest of the book is written in Hebrew. There are some different explanations as to why there is this difference in language. Dr. Andrew E. Steinmann in Concordia Commentary: Daniel suggests that it is another tool in the author’s toolbox to relay his message. When Daniel talks about new for the other nations, he uses the language of the other nations. Even with the shift in language, Steinmann points out that Daniel writes a unified book in message and style.
As a quick side note, it’s important to note that the detail about a unity in message and style are important when considering the additional sections of Daniel that are incorporated into the canon of the Roman Catholic church. Churches outside of Rome don’t incorporate those sections because there isn’t Hebrew and Aramaic textual support for their inclusion. What we have are from Greek translations rather than the original language. While the Greek seems to be translated from a Semitic language, we don’t have evidence of those sections in the earliest manuscripts. Also, the style and tone are rather different compared to the rest of the book that they appear to be a later addition.
Now, back to the main point of this post. In People’s Bible Commentary: Daniel we read this:
Hebrew was, of course, the language of the Jews, and those portions of Daniel’s book intended especially for the Jews were given to them in their language. Aramaic, a member of the Semitic family of languages and thus closely related to Hebrew, was the official language of much of the Newar East at the time of Daniel. It was the language of diplomacy and of commerce in the ancient world, just as English is the “universal” language today. Those portions of Daniel’s prophecy that speak of God’s judgement on the world powers were written in Aramaic, the language which the nations of the world could understand. p. 10
Like Steinmann, we hear a similar idea suggesting that the language difference is a tool to get the message across to those hearing the words of this book.
There are some who suggest that there were Hebrew and Aramaic versions of the text as the people of Israel were switching languages during this period from Hebrew to Aramaic and that today we have portions of the Hebrew and portions of the Aramaic copies.
Most of what I could find suggested the language differences to be a tool rather than a flaw in the text or varying authorship. However, Daniel identifies himself as the author at various points (8:1, 9:2). Also, Jesus makes reference to Daniel in Matthew 24:15. This would lead us to suggest that Daniel is the author and that his use of language was a tool rather than a flaw.
I hope that helps a little.
If you have any questions or want to dig deeper, please let me know.
Blessings on your day!