Add multi-language support for email templates and API error messages

[nanaf6203-bit]

Problem

All email templates and error messages are English-only. User model has languagePreference field but it's unused.

Expected Outcome

Internationalized email templates and error messages with English and Spanish support.

Acceptance Criteria

• i18n module integrated
• Email templates support multiple languages
• API error messages translated to user's language
• Spanish translations for email templates

Files Affected

New src/i18n/, src/email/templates/, src/main.ts

Difficulty

Medium

Estimated Effort

Medium (1 week)

Labels

i18n, enhancement, feature

[monique-7arch]

@monique-7arch has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi, I'd like to work on this issue.

I have hands-on experience with backend development, databases, and API design, as well as frontend work with React and modern CSS. I write tested, maintainable code and enjoy contributing to open source projects.

I've reviewed the issue and I'm confident I can implement a solid solution while following the project's conventions.

Happy to get started right away if assigned — looking forward to contributing.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @monique-7arch to this issue.

[drips-wave]

Congratulations, @monique-7arch! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @monique-7arch: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[drips-wave]

This issue has been completed by @monique-7arch as part of the Stellar Wave Program's 7th Wave 🥳

😎 @monique-7arch: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

🧑‍💻 Repo maintainers: How'd the contributor do? Leave a review to share your experience working with them.