Articles.
Five locales, one codebase
Localization belongs to the field, not the deployment. Mark what varies, leave the rest alone.
Pages and blocks: one field decides
A type is a page because its model contains a slug field. Nothing else marks the difference — and that turns out to be enough.
Previewing the real page, not a drawing of it
Most headless previews render their own approximation of your site. Here is what changes when the preview is the site.