The translation is likely protected by copyright (but the scope of protection may be relatively thin because the range of expression for a translation may be narrow or limited). When you say "incorporate", do you mean copy? If you are commenting on the translation, that might be considered fair use (which is often a fact-specific inquiry). Copying short excerpts necessary for your commentary also may be fair use (but whether it is or not would depend on the specific facts). For specific legal advice, you should consult with a copyright attorney in private. If your book is to be published, it should be vetted by an attorney for these and other issues.
Below is the section of US Copyright Law regarding "fair use":
17 USC §107: Limitations on exclusive rights: Fair use
Notwithstanding the provisions of [17 USC § §106 and 106A], the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include--
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.
This answer is for informational purposes only. It is not intended as specific legal advice regarding your question. The answer could be different if all of the facts were known. This answer does not establish an attorney-client relationship.
You probably need permission.
I like to describe it as there are layers of copyright. The first layer is the copyright in the original work, which you say has expired. The rules on expiration of copyright are very complex and vary from country to country so be sure you have this right. Then on top of that there is a layer of copyright in the translation. To the extent that the translator added creative material, it is protected by copyright. That covers most of the translation. Whenever a translator makes a choice of which English words to use, that is creative expression. Generally you can not copy the translator’s work without permission. If the translator worked from another translation, not from the original work, there may be an intermediate lawyer of copyright as well.
You mention fair use. That is also a complicated concept. Essentially if you copy a very small amount of the work, you are probably ok. But if you copy any substantial amount you are not. And that is only a rough rule of thumb. You will get a much more accurate answer if you consult a copyright attorney.
This answer is not intended to be a substitute for personalized legal advice. I have presented only an overview of the legal issues. There are many nuances and some exceptions to general legal principles. Real problems are very fact based.. If you have a specific legal problem, you should consult an attorney. You can find more useful information on my law firm website resources page at www.marshallcomputer.com or you can contact me directly at email@example.com or (206) 524-0655.
LIterary translations carry separate copyright with them, Contact the publisher of the translation to seek permission. If it is simply an excerpt, and not a high-profile text, the publisher (or the translator if he/she retained subsidiary rights) may entertain a request to use the material gratis, provided that credit is given. Good luck.