Internationalisation (i18n) / localisation pipeline

[PierreRaybaut]

Summary

Bring DataLab-Web to feature parity with DataLab Qt on translations: a working i18n pipeline covering the React UI, the Sigima labels surfaced through Pyodide, and the result formatting (units, dates, numbers).

Motivation

DataLab Qt ships French and English translations end-to-end (UI strings via gettext, computation/result labels via Sigima's _() wrapper, documentation via sphinx-intl). DataLab-Web currently has no translation layer: every label is hard-coded in English.

Architectural impact

• Two translation domains to bridge: React/TS strings (probably FormatJS / LinguiJS / a thin t() helper) and Sigima Python strings already wrapped with _() but with no .mo loader inside Pyodide.
• Bundle strategy: pre-bundle .mo files in the Vite build vs lazy-loaded per locale; impacts startup size.
• Runtime locale switching: Sigima's gettext lookup is process-global — switching language at runtime requires re-instantiating translation objects on both sides and re-rendering the React tree (including auto-generated DataSetForms, which currently snapshot labels at mount).
• Result formatting: numbers, units, dates rendered in tables and plot annotations need a locale-aware formatter (probably Intl.*); coordinate with Sigima's existing formatting helpers.
• Documentation: a separate sphinx-intl pipeline for the user guide, mirroring DataLab desktop.

Acceptance criteria

• A t() helper (or equivalent) covers every user-facing TS string; ESLint rule (or grep guard) flags hard-coded literals in src/components/.
• Sigima .mo files are loaded inside Pyodide and a language switch in the UI propagates to DataLabRuntime (re-runs bindtextdomain and refreshes cached catalogs).
• At least French and English are shipped, with a complete UI catalog and a Sigima catalog re-using the upstream .po files.
• Documentation builds a localised HTML target (mirrors DataLab desktop's pipeline).
• An E2E test asserts that switching language refreshes both UI strings (a menu label) and a Sigima-driven label (a parameter dialog title).

References

• DataLab desktop pipeline: DataLab/.github/copilot-instructions.md → "Translations" section
• Sigima: Sigima/sigima/locale/