1) The last version is the 1.5.1 and it was released in 2017. Is it the plugin abandoned? Are you planning to release a new version soon or the plugin is just perfect and doesn't need new updates?
It has, indeed, been a while since the last release but please let me reassure you that Juicebox is still actively being developed.
The current version is certainly quite stable but, having said that, the next version will likely be a bug-fix version, rather than introducing new features.
Unfortunately, I do not know when the next version will be released (there is no set release schedule) so any estimate I give you could turn out to be wholly inaccurate.
2) Is it possible to use it only in wp posts or also in wp pages? (now I'm using the lite version and I can see the JuiceBox Button only when I'm creating new posts)
Yes. WP-Juicebox can create galleries in pages as well as posts. The 'Add Juicebox Gallery' button should appear in both 'Posts -> Add New' and 'Pages -> Add New' (and it does in my own WordPress v5.2.3 installation).
You might need to temporarily revert to a default theme (such as Twenty Nineteen) and disable your active plugins one-by-one (checking for the 'Add Juicebox Gallery' button in 'Pages -> Add New' after each one) to see if you can find a conflicting theme or plugin.
3) Using Divi theme and Polylang plugin for multilanguage, when I create a new post, after the translation in another language, the gallery disappears and I have to save it again to show it. Is it a bug?
I'm not sure what could be causing this problem as, unfortunately, I do not use the Divi theme or the Polylang plugin.
The first thing to do is to try to determine whether it is the theme or the plugin which is causing the problem so please try to reproduce the problem under a default theme (such as Twenty Nineteen). This might at least rule out the theme as being a factor in the equation.
If the problem still occurs, then please post back with a detailed list of steps that I can use to try to reproduce the problem myself (as I am not familiar with Polylang).
Once I am able to reproduce the problem, I should be able to troubleshot it and hopefully find a solution.
Also, if the problem manifests itself in your WordPress frontend, then please post back with a link to your gallery's web page so that I can see the problem for myself within your own Wordpress site. (If I can see a web page where the gallery should appear, then I might be able to determine why it does not appear.)
If all else fails, you can always embed a Juicebox gallery manually into a WordPress page or post using the baseUrl method documented here.