The game is very nice compared to the original engine, but... even though I installed the Italian version (and I imagine I have the same problem with languages ​​other than English), the menus, objects, etc. remain in English, while only the dialogue is translated into Italian.
Is this correct?

same issue with plolish language. To be honest if the mod has limitations while adding translations then in my opinion these translations should not be included in the first place. What's the point of adding translation if only half of the stuff is translated

It's actually strange!
For example, there's another MOD called DevilutionX, which is fully localized.
This Belzebub, however, is interesting because it significantly extends and expands the game, as well as modernizing its graphics.
It's a shame about the partial localization.

The weird thing is there are fan made translation that ran in the vanilla build of the game with everything translated.

Ich suche deutsche Sprachfiles für meine GOG Version von Diablo 1 und Hellfire.
Kann mir hier irgendwer weiterhelfen?

Read this
https://www.gog.com/forum/diablo/diablo_gog_german_patch

The fan translations required hacking the client and replacing graphics and could break multiplayer comparability. Since they where hexedited in to the .exe it wasn't easy to reuse them elsewhere and additionally they where under the extra restriction of having to phrase in fewer letters then the English version which could lead to some odd translations.

Diablo was never build with translation support, that was changed for the PS1 port though that wasn't easy to provide fan translations for since it would require a game backup with modified data.

For DevilutionX we implemented full support for the gettext format and then had the community help with providing new translations for the full game and expansion. The nice thing here is that the language can be changed at runtime so you don't need to install different builds.