Ensure client don't cache plugin translations file

[kontrollanten]

Describe the problem to be solved

Currently plugin translations are served from /plugins/translations/{locale}.json There's no Cache-Control header nor content hash in the URL, which may cause clients to unexpectedly cache the file with stale content.

Describe the solution you would like

Since translations are always loaded they can be served direct with the HTML file. If that's not possible, add a content hash to the URL and set Cache-control max-age to one year.