Answer
Start from the "Translatable Texts" screen, not "Text Translations" directly — Translatable Texts lists every translatable item regardless of whether a translation exists yet, filtered by module CONFIG, a Client-Configuration main type, and the User layer, with the item's path telling you its configuration context and client model. From there, open Text Translations for that specific item and add the translation. Critically, there's a known ordering gotcha: changing configuration in Page Designer invalidates the metadata cache, but adding or changing a translation does not — so the sequence has to be make the translation change, then refresh the language cache, and only then revisit the page, otherwise the page you (or anyone else) load next will still show old translations even though the metadata itself is current; if it's still not showing after following that order, unpublishing and republishing the page from Page Designer forces both caches to resync.