The paragraph language selects the CJK typesetting rules (line breaking,
punctuation spacing, the default emphasis position and localized glyphs),
but plugins could not read or set it. `lang` holds it as a BCP 47 tag, as
the HTML `lang` attribute does; `null` or `''` removes it.
On a text shape it applies to every paragraph and reports 'mixed' when
they differ. On a text range it applies to the whole paragraphs the range
touches: `update-text-range-attrs` now writes paragraph attrs such as the
language there instead of copying them into spans. The tag pattern moves
to `schema:text-language`, shared by the content schema and the plugin
validator.
AI-assisted-by: claude-opus-5-5
2d56f26d5f