systemLocaleFormatter

A LocaleFormatter for whatever locale the device is currently set to.

Opt-in, and deliberately not the default. A2uiRenderer keeps FallbackLocaleFormatter until a host passes something else, because a formatter that reads the device makes the four functions depend on the environment: the same payload and the same test then produce different strings on a developer's machine and in CI. A host that wants the device's locale is choosing that, in one line, where it can be seen.

The locale is read once, here. A device whose locale changes while a surface is on screen keeps formatting with the old one until the host builds a new formatter.