feat(host): add getLocaleProvider for the host's selected language

[kalininilya]

Adds getLocaleProvider, so a product renders in the language the user picked inside the host rather than inferring one from navigator.language — which reports the operating system's preference and is wrong whenever the two differ.

import { getLocaleProvider } from "@parity/product-sdk-host";

const provider = await getLocaleProvider();
const sub = provider?.subscribeLocale((locale) => {
  i18n.activate(SUPPORTED.has(locale.languageTag) ? locale.languageTag : "en");
});

subscribeLocale fires with the current locale and again on every change; the returned HostSubscription carries unsubscribe and onInterrupt. getLocaleProvider resolves to null outside a host container.

languageTag is a BCP 47 tag such as "en", "pt-BR" or "zh-Hans". The set is open — a host adds languages without an SDK release — so a product that ships no catalog entry for the tag it receives picks its own fallback.

The fake client models locale as not-modeled, so a test that reaches for it fails loudly rather than silently returning nothing.

Protocol counterpart: paritytech/host-rust-core#526.

Blocked on

A @parity/truapi release carrying the locale domain. The catalog pins ^0.10.0, which predates it, so CI will not go green until that lands.

Verified

• pnpm --filter @parity/product-sdk-host typecheck against a local @parity/truapi built from RFC: Host locale subscription host-rust-core#526 — no locale errors. One unrelated error remains (system.info / getProductContext missing from the fake client), which reproduces on a clean tree with the same link: it is drift between host-rust-core main and the published 0.10.0, not this change.
• pnpm --filter @parity/product-sdk-host test — 20 files, 123 tests passed, including the new in-source locale.ts test.

[duewarn]

Hey @valentinfernandez1 @decrypto21 @TarikGul @pgherveou ! As paritytech/host-rust-core#526 has been approved and merged, could you take a look at the implementation for the Product SDK? This functionality is included in the first release for mobile apps and the Funding modality.

[valentinfernandez1]

Hi @kalininilya will take a look. There are some failing checks that you need to resolve. <

Also, how urgent is this? to know whether we need to create a truapi release asap to include the pr from host-rust-core or if it can wait till it lands alongide the next release.

[valentinfernandez1]

Looks good, blocking until truapi release and fix on the changeset are fixed

[duewarn]

@valentinfernandez1 thank you for review! These changes have not yet been merged on the hosts - this is expected to happen today or tomorrow. However, as soon as you release the new version of the Product SDK, it will be possible to integrate the new subscription into GetCash (Funding modality), which is included in the scope of the first release on 4 September.

[valentinfernandez1]

Ok I will do a release on truapi first so we can integrate this change.

[valentinfernandez1]

Truapi release done, bumped by #353. I'm also bumping truapi on dotli with #189 side so this should be available there on the next dotli release too.

[github-actions]

📦 Bundle size impact

Comparing 2026-08-31T16:06:14.859Z → 2026-08-31T16:05:56.393Z

	Package	Entry	Bundled before	Bundled after	Δ	Ship gzip Δ	Shake ratio
🟢	@parity/product-sdk	./host	98.1 KB	98.3 KB	+159 B (+0.2%)	0 B	8% (was 8%)
🟢	@parity/product-sdk-host	.	98.1 KB	98.3 KB	+159 B (+0.2%)	+36 B	8% (was 8%)
🟢	@parity/product-sdk-statement-store	.	109.8 KB	110.0 KB	+159 B (+0.1%)	0 B	9% (was 9%)

Thresholds — 🟡 ≥10% or ≥5.0 KB · 🟠 ≥20% or ≥15.0 KB (bundled). Percentage only applies once the baseline is ≥ 10 KB. Informational — this check never blocks merge.