feat: backend i18n — serve translated component metadata via Accept-Language

[RamGopalSrikar]

Summary

Backend component i18n (original scope)

• Backend locale files — src/backend/base/langflow/locales/ with 4,748 flat keys across 359 components (Tier 1: component display_name/description; Tier 2: input/output field display_names)
• Accept-Language middleware — set_locale in main.py parses the header once per request and sets request.state.locale; /api/v1/all calls translate_component_dict() for non-English locales, leaving the English fast path untouched
• GP integration scripts — extract_backend_strings.py (with --check CI flag), upload_backend_strings.py, download_backend_translations.py using a separate langflow-backend GP bundle
• CI workflows — gp-backend-check.yml (PR validation + auto-commit on non-fork PRs), gp-backend-upload.yml (extract → upload on release-* branches)
• Frontend — useCustomApiHeaders sends Accept-Language; language change clears useTypesStore and invalidates ["useGetTypes"] so the sidebar refreshes without a page reload
• Canvas sync — syncNodeTranslations() in flowStore patches all canvas nodes whenever the types catalog is refreshed

Starter flow metadata i18n (new)

• Auto-extraction — extract_backend_strings.py now auto-discovers all initial_setup/starter_projects/*.json files and emits starter_flows.{safe_key}.name / starter_flows.{safe_key}.description keys; no manual work needed when new starter flows are added
• translate_starter_flows() — new function in utils/i18n.py mirroring translate_component_dict(); derives a stable name_key slug from the English name and attaches it to each FlowRead
• FlowRead.name_key — stable locale-independent slug exposed on the response model so the frontend can match flows by identity regardless of the active language
• read_basic_examples() refactor — accepts Request, caches raw flow_reads list (not compressed response), translates per-request via locale from middleware; refetch triggered on language change via i18n.language in React Query key

Templates modal & empty page i18n (new)

• Sidebar title, category group labels, nav items, card category badges, search placeholder, empty-state messages, "Start from scratch" footer, "Blank Flow" button
• Empty project / Start building / "Begin with a template" / New Flow on emptyPage and emptyFolder
• All 7 locale files updated (en, fr, de, es, ja, pt, zh-Hans)

Depends on: feat/globalization-pipeline (frontend i18n strings) — please merge that PR first.

Architecture

User selects language
  → Accept-Language: fr on every request
  → set_locale middleware → request.state.locale = "fr"
  → /api/v1/all: copies cached English dict + substitutes component strings from fr.json
  → /api/v1/flows/basic_examples/: translates starter flow name/description from fr.json
  → frontend renders translated strings; name_key used for stable flow matching
  → syncNodeTranslations() patches existing canvas nodes with fresh translated metadata

Test plan

• Start backend + frontend; switch UI to French — sidebar component names appear in French
• Switch back to English — sidebar reverts without page reload
• Open Templates modal in French — sidebar labels, card badges, search placeholder, footer all in French
• Starter flow cards (Basic Prompting, Vector Store RAG, Simple Agent, Market Research, etc.) show translated names and descriptions
• "All templates" grid shows translated flow names and descriptions
• Empty project page shows translated strings
• Open a flow with nodes → change language → canvas node titles update to new language
• Unknown locale (e.g. Accept-Language: xx) falls back to English
• Run python scripts/gp/extract_backend_strings.py --check — exits 0 (en.json in sync)
• Adding a new starter_projects/*.json → re-run extract → new keys appear automatically

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Replaces #12258.

[Cristhianzl]

lgtm