[mostafaelqaseir]

Arabic interface, right to left

Adds Arabic as a built-in interface language and makes the player and the contents page read right to left for it, without changing what English (or any other language) renders.

What changes

• Interface text. An ar block in TRANSLATIONS (engine.js, contents.js) with Arabic's plural forms (one, two, few 3–10, many 11–99, other). <html lang="ar"> sets dir="rtl" unless the page sets its own dir (RTL_LANGUAGES, one list in each file, kept in step like the translations).
• Right to left, by logical properties. The left/right rules on cards, help, the finish screen, the contents and the unit art become logical (border-inline-start, inset-inline-end, padding-inline-start, text-align: start/end). In left-to-right pages they compute to the same values, so English is unchanged; in Arabic the cards, captions, help, cover and contents mirror.
• What stays left to right. The stage, the player bar and mathematics (svg.m expressions and .bline answer lines in cards). Drawings keep their coordinates, seeking and ←/→ mean the same in every language (as YouTube does in Arabic), and 3 + 4 = [box] keeps its order, as in Arabic school books. Without this the first quick check rendered its equation scrambled.
• Arabic type. Arabic letters join, so letter spacing is dropped on the uppercase kickers. Faces are each system's own Arabic UI and book faces (Segoe UI, SF Arabic/Geeza Pro, Noto); no font file is added to the books. A book's own book.css / theme still wins.
• UI follows --ui. The engine's canvas measuring (and stage text) used a hard-coded face list; it now reads --ui, so an Arabic page (or a book that sets --ui) measures in the face it shows. The value is normalised so English markup stays byte-identical.
• Narration. tts.py: ؟ ends a caption as ? does. I ran Edge's ar-SA-HamedNeural and ar-SA-ZariyahNeural live: every word boundary matched the text as written, with diacritics (إِنَّ), Arabic-Indic digits (٥) and inline maths (x + 3 = 7), so [[marks]] need no special handling. An old edge-tts gets HTTP 403 at the handshake; uv run --with edge-tts (as documented) takes the current one.

Checks

• engine/check_i18n.py: new Arabic lesson and Arabic book checks (dir, LTR stage/bar, plurals, numbers, a key on an Arabic layout, the card's accent edge on the right, the equation's order, the step counter's order, the finish card, the cover arrow, contents and progress). "English unchanged against main" still passes, byte for byte.
• .github/scripts/check_tts_text.py (new, stdlib only, added to CI): captions and marks from narration text with hand-written word timings, Arabic and English, no network.
• engine/check_tapwords.py: the interface-strings check covers every language in TRANSLATIONS instead of a fixed list.
• check_skill.py, check_site.py (engine copies synced with engine.py site), ruff, check_latex.py, check_tapwords.py, check_volume.py: all pass.

Docs

core/engine.md (what RTL does and keeps), core/publish.md (built-in languages), core/sound.md (Arabic voices and what was checked), lessons/diagrams.md (an Arabic page reads from the right, its maths from the left).

Not in this PR

• Arabic-Indic digits in num() and answer boxes (digits stay 0–9 with a point).
• eleven.py alignment for Arabic (not tested).
• An Arabic book on the shelf (no copyrighted material, per CONTRIBUTING).
• Books that install the Skill without the repository get this with the next engine-v* release.

🤖 Generated with Claude Code

https://claude.ai/code/session_01SzbEL6yNpeuSXaqqgkchqD

[DozenTwelve]

Thank you, Mostafa! Merged and live at https://papermorph.diamonddoge.org/. A careful piece of work: logical properties so English stays byte-identical, the stage and maths kept left to right, and checks for all of it. Much appreciated.