locale.t() needs a key-level fallback — WCAG 4.1.2 gap

[johnleider]

Problem

After 9823d09b (triage row B07), six components route aria strings through locale.t():

• Dialog/DialogClose.vue — locale.t('Dialog.close')
• AlertDialog/AlertDialogClose.vue — locale.t('AlertDialog.close')
• Snackbar/SnackbarClose.vue — locale.t('Snackbar.close')
• NumberField/NumberFieldIncrement.vue — locale.t('NumberField.increment')
• NumberField/NumberFieldDecrement.vue — locale.t('NumberField.decrement')
• Rating/RatingRoot.vue — locale.t('Rating.valueText', { value, size })

If a consumer hasn't registered locale messages (and v0 ships none by design), locale.t('Dialog.close') returns the literal string "Dialog.close" as the aria-label. That fails WCAG 4.1.2 Name, Role, Value — assistive tech announces the key identifier instead of a human label.

The same hole applies to createLocaleFallback() (the no-adapter path), which also returns the key verbatim.

History

Commit 245182f0 (2026-03-20, `feat(useLocale): change t() to variadic signature`) removed the `fallback?` parameter from `t()` to align with Vuetify's `t(key, ...params)` convention. The BREAKING CHANGE note advised consumers to use the fallback locale mechanism or ship default messages — but v0 itself ships no defaults and components have no way to contribute their own.

Rejected approaches (2026-04-20)

• Re-add the fallback param (`t(key, params?, fallback?)`) — reverses the 2026-03-20 BREAKING CHANGE, re-introduces divergence from Vuetify's signature.
• v0 ships minimal default English messages at plugin install — violates headless minimalism; v0 would own strings it currently doesn't.

Angles worth exploring

• Component-level `ariaLabel` prop as the fallback boundary, so consumers supply English once without wiring up `useLocale`. Fallback chain becomes `prop ?? locale.t(key) ?? ???`.
• Missing-key handler on the adapter — `createLocale({ onMissing: (key) => ... })`. Pushes the decision to app layer.
• A `@vuetify/v0/messages/en` subpath export that consumers can `app.use(createLocalePlugin({ messages: { en } }))` — headless by default, but defaults are reachable without re-implementing them.

Tension

Three competing pulls, all load-bearing:

1. Vuetify signature alignment (for direct adapter migration)
2. Ergonomic safety for unconfigured consumers (WCAG 4.1.2)
3. Headless minimalism (v0 ships no strings)

Today (1) and (3) are satisfied; (2) is the gap.

Acceptance

An approach that closes the WCAG gap without reverting (1) or violating (3). The two rejected options above are off the table.