get multilingual settings and available languages from backend

[erral]

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This supersedes #5506

@sneridagh as commented, I have created a new PR with the changes.

We have 2 things (at least), to address:

• The @addons endpoint is for Managers only, so we would need to get the isMultilingual to the @site endpoint.
• How do we configure Volto to get the translations expander in each content request? Right now we do this in the getContent action getting the value from settings. I don't know if querying the redux state in the action body is acceptable at all.

Anyway, this is open to discussion.

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[sneridagh]

@erral oh I was unaware of that. Let's put the multilingual check in the @site endpoint.
I might look into it this weekend.
Regarding the state lookup in the action, it can be done, and it's acceptable. It's the use case that an action depends on another action (a thunk), but the request would be in there already (I guess). We can try.

[pnicolli]

@erral Do you see any issues in adding this to Volto 18? Is it breaking if sites don't have the latest restapi version? As discussed in the Volto Team Meeting today, the #6807 PR is going in 19 so maybe this one should go in Volto 19 as well?

[erral]

Right now this PR requires latests plone.restapi (9.12).

But I hav no problem on waiting to 19, and we can include there these and the Pr you mentioned.