There is no way to configure the homepage(s) target for secondary languages via the back-office.
The only way I found to achieved this is by importing a language/[language_abbr]/system.site.yml.
Default configuration :
Available translation :
It should be great to be able to manage this setting translation (and also the others on this page) via the back-office.
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.
I had a look in to this and can see the issue is with translating any path data type, not limited to the front page or other paths in system.site config. I've started an MR with a proof of concept to demonstrate what's missing. Note we possibly need a separate FormElement for paths, not sure.