feat: add Spanish localization and language-aware prompts

[Dragost]

Description

Adds Spanish (Spain) as a first-class DeepTutor locale across the backend, web interface, and model-facing prompts.

• Introduces extensible backend and frontend language registries.
• Adds es / es-ES to the language selector and a complete Spanish UI catalog.
• Separates the interface language from the default model response language, with backward-compatible migration.
• Localizes backend status messages, metadata, quiz-judging prompts, MCP services, CLI apps, and context-budget UI.
• Preserves the selected response language across agent and model calls, explicitly requesting Spanish from Spain.
• Adds locale key/placeholder parity and Spanish translation-quality regression tests.

Related work

Includes the UI/model language separation proposed in #758, adapted to this Spanish-localization branch. The corresponding commit retains its author as co-author.

Module(s) Affected

• agents
• api
• config
• core
• services
• tools
• utils
• web (Frontend)
• scripts
• tests

Checklist

• I have read and followed the contribution guidelines.
• My code follows the project coding standards.
• I have run all pre-commit hooks on the files changed by this PR and fixed any issues.
• I have added relevant tests for my changes.
• Documentation updates are not necessary for this change.
• My changes do not introduce any new security vulnerabilities.

Validation

• Targeted backend localization/settings/prompt tests: 62 passed (2 warnings).
• Web Node tests: 455 passed, 0 failed.
• All pre-commit hooks passed on the 45 files changed by this PR.
• The repository-wide pre-commit run --all-files was also checked: every hook except Prettier passed; Prettier only attempted to reformat 41 files already present on the base branch and not touched by this PR. No unrelated formatting changes were included.
• i18n catalog key and placeholder parity passed.
• ESLint passed with 0 errors (56 existing warnings remain).
• Next.js production build and TypeScript passed; all 58 static pages were generated.
• git diff --check passed.

Additional Notes

The branch is rebased onto current dev commit 96d6df9e (v1.5.13), is conflict-free, and GitHub reports it as mergeable. The Spanish catalog now covers the latest upstream starter suggestions, Tencent IMA, GraphRAG compatibility, and book generation/reading flows.

The three commits intentionally separate backend locale infrastructure, the web locale/catalog, and response-language propagation through model prompts. The rebase preserves the expanded upstream book-language selector while keeping interface language and model response language independently configurable. The Spanish catalog uses Spain-specific terminology and a consistent informal tú voice.

[Dragost]

Rebased this PR onto the current dev branch and completed the Spanish coverage for the newly added MCP services, CLI apps, and context-budget UI (145 frontend strings and 16 backend messages). I also wired locale key/placeholder parity into the Web Node test command.

Validation after the rebase: 369 Web Node tests passed, TypeScript passed, 3,560 Python tests passed (7 skipped), Ruff passed, and all project pre-commit hooks passed on the changed files.

[Christian-B-Koller]

I am waiting for your changes to be integrated into main. I have done a German language translation with every text on an older version of DeepTutor. I I'll wait until #728 lands and then move my translation over to the architecture and code that is prepared here.

[ting-hong-shieh]

Hi @Dragost — I noticed that GitHub currently marks this PR as conflicting with the latest dev branch. I’m working on zh-TW support that would build on the language registries introduced here, so I’d be happy to help with the rebase conflicts if that would be useful.

I can reproduce the rebase against the current HKUDS/dev, run the relevant Web and backend i18n tests, and provide either a cherry-pickable commit or a PR to your fork—whichever you prefer. I won’t make changes until you confirm, to avoid duplicating work if you’re already handling it.

[Dragost]

Thanks, @Christian-B-Koller, and thank you as well, @ting-hong-shieh, for offering to help.

I’ve now rebased #728 onto the latest dev, resolved the conflicts, and updated the Spanish catalog with the new CodeBuddy, model synchronization, and Codex reasoning strings introduced upstream. The PR is conflict-free and fully up to date now, so no additional cherry-pick or PR is needed at the moment.

We’re currently waiting for a maintainer to review it and authorize the fork workflow. Once merged, the language registries and the separation between interface and model-response languages should provide a clean base for both the German and zh-TW translations.

Thanks again to both of you for your work and willingness to contribute!

[Dragost]

Update: this PR has now been rebased onto current dev (96d6df9e, v1.5.13) and is conflict-free/mergeable again.

The BookCreator conflict was resolved while preserving the expanded upstream book-language selector. I also synchronized the Spanish catalog with the current English source: 121 new strings were translated and 3 obsolete provider-specific keys were removed. This covers the new starter suggestions, Tencent IMA, GraphRAG compatibility, and book generation/reading flows.

Current validation: 62 targeted backend tests passed, 455 Web Node tests passed, all pre-commit hooks pass on the 45 PR files, ESLint reports 0 errors, the Next.js production build and TypeScript pass, and i18n key/placeholder parity is clean. The history remains split into the original three focused commits.

Ready for maintainer review. Thanks again to everyone coordinating downstream locale work around this foundation.

[Christian-B-Koller]

Thank you very much @Dragost for your work, I am looking forward to the approval of the maintainer.