Fill missing es/pt/de/zh translations across frontend and backend

[Irozuku]

Summary

All four non English locales (es, pt, de, zh) are now at 100% translation coverage, both on the frontend and the backend. This was purely missing data, no logic was changed.

On the frontend, every i18next key present in en now exists in es, pt, de, and zh. On the backend, every MultilingualString(...) literal now carries es, pt, de, and zh keyword arguments alongside en. A static AST based checker script was also added so future gaps can be caught without running the app.

Type of Change

• Backend change
• Frontend change
• CI / Workflow change
• Build / Packaging change
• Bug fix
• Documentation

Changes (by file)

Frontend - locale JSON files

• src/utils/i18n/locales/de/common.json, datasets.json, explainers.json, models.json: fill missing de keys
• src/utils/i18n/locales/de/hub.json: new file, full hub namespace translation for de
• src/utils/i18n/locales/pt/common.json, datasets.json, explainers.json, models.json, etc.: fill missing pt keys
• src/utils/i18n/locales/pt/hub.json: new file, full hub namespace translation for pt
• src/utils/i18n/locales/zh/common.json, datasets.json, explainers.json, models.json, home.json, hub.json, generative.json: fill missing zh keys
• src/utils/i18n/locales/es/models.json: add missing _many plural forms
• src/utils/i18n/locales/pt/models.json: add missing _many plural forms
• front/i18next.config.js: add pt, de, zh to the locale list so i18next-cli status reports them

Frontend - components

• src/components/tour/NavbarTourButton.jsx, TourButton.jsx: replace hardcoded "Start Tour" / "No tour available" strings with t() calls
• src/utils/i18n/locales/*/common.json (all locales): add startTour, noTourAvailable, tourNotAvailable keys

Backend - MultilingualString coverage

• DashAI/back/models/scikit_learn/*.py (32 files): add zh, de, pt kwargs to all MultilingualString calls
• DashAI/back/models/hugging_face/*.py (21 files): same
• DashAI/back/models/*.py (base_torchvision, cnn, efficientnet_b0, lenet5, mlp, resnet18, resnet50): same
• DashAI/back/dataset_sources/*.py, dataloaders/classes/image_dataloader.py, exploration/explorers/*.py: same

Tooling

• scripts/check_translations.py: new AST based script that scans all MultilingualString literals and reports coverage per locale; supports status (table) and lint (exit 1 on gaps) modes

Testing

• npx i18next-cli status (run from DashAI/front) reports 100% for all four locales
• python scripts/check_translations.py reports 100% for all four locales across 1341 backend literals