ConstReadonlyActive: "language"The user's UI-language override (null = no preference).
Owned via usePersistentState against the surrounding PersistentStateProvider (Studio, or a
standalone host) — the mirror of ThemeStateKey in @sheetxl/react. A host that wants to read
or seed it (e.g. the VS Code extension, whose store bridges to the extension host's state) does
so with the same key, or by supplying its own provider/store. The value is a BCP 47 tag; absent
means "no preference" and the pack service default stands.
Persistence key for the user's UI-language preference.