A language preference: automatic, or one BCP 47 tag.
The authority validates the shape of a tag and never resolves it: the closed set of languages lives with each consumer (koru-i18n for the UI, the speech engine for dictation, the spell engine for spelling), and pinning a copy of any of them here would be a second list to keep true. An unrecognized tag resolves to automatic at the consumer.
Grew out of the dictation preference when spell checking needed exactly the same shape; [DictationLanguage] remains as an alias so nothing renames on the wire or at a call site.