Talk to a library, especially if you are in a metropolitan area. Libraries frequently have a preservation or archive department or know of another library with one. They will assess it and store it in a proper environment to maintain the journal as well as add it to a collection so others can read it as they wish.

Absolutely yes, it is of interest to researchers. As to who might take it, subject matter experts might have more thematic ideas, but start by contacting your local historical society, or public library system or university library or history department. In the US the army historical command or the library of Congress manuscript division might be interested as well.

I wanted to chime in with the fact that you should absolutely do what you can to make sure this is preserved and available because due to a multitude of reasons there is a bit of a dearth of first person primary accounts of fighting on the Eastern Front for the USSR. Not that there’s nothing but it hasn’t quite had the same history as say memoirs of WWII on the American side have had. Making it all the more important to try and make available what there is. And you should definitely talk to a local historical society. I don’t know if you’re in the US or somewhere else and the answer of where to take it could be slightly different outside of the US but I’m pretty sure just about everywhere has an historical society of some kind and they’d be good people to touch base with.

I have some more specific guidance for getting it in the right hands, and eventually accessible to researchers who could find valuable information in it.

Firstly, don't just bring it to your local library. Don't get me wrong, public libraries are fantastic resources, but most do not have robust archival preservation programs, or communities of scholars who would be interested in your uncle's journal and able to discover it using the local library catalog. The chances of it getting digitized and put online so it's accessible to more people are also slim at a local library, although this of course depends on where you are.

First, research libraries and archives in your country that specialize in Soviet/Russian history or the history of World War II more broadly. If you're not sure where these places might be, you can research universities with strong history departments, particularly, Slavic or Russian studies programs, and then check their libraries and archives. Alternately, check prominent journals in the field and see if they're affiliated with a university.

Make a transcript of some key passages and take photographs of a few pages of the diary, including the front cover and a page with your uncle's name on it, preferably his signature. Any details about his unit and where he served should be documented photographically. Send this to the curator or archivist at the institutions you've identified, along with the transcripts of some of the more important passages. This gives them an idea of the content of the diary. The photographs prove that you're serious (libraries sometimes get prank solicitations like this), the diary appears to be authentic, and begins to establish provenance.

Depending on the rarity of this kind of document, you might be able to get monetary compensation, either directly from the library or from a book dealer who will buy it from you and then sell it directly to a library. (Although keep in mind that dealers also sell to private collectors, so if you care a lot about it being publicly accessible, a library is your best bet.)

Before you can sell or donate the work, you will need to verify that it is legally yours. You will sign a document basically saying that the item is yours to give, and clarifying things like copyright. Some deeds of gift get quite specific about how soon the item will be cataloged or digitized, but that's not common.

While you're sorting all this out, you should keep the diary in a cool, dry place (not an attic or basement) out of direct sunlight and preferably in an acid-free box or envelope. If the diary is fragile or has a weak binding, you should handle it as little as possible.

I hope this is helpful!