feat: implement i18n support for English, Hindi, Spanish, and French (#679)

[knoxiboy]

Description

Resolves #679

This PR updates the translation and localization system to support English, Hindi, Spanish, and French. It replaces the previous Telugu and German dictionary entries with comprehensive Spanish (ES) and French (FR) translations, and updates the header dropdown selector dynamically.

What changed

• Replaced German and Telugu translation dictionary entries with Spanish (es) and French (fr) in src/i18n.js.
• Updated the header language dropdown options inside index.html to offer EN, HI, ES, and FR options.
• Retained the automatic DOM translation traversing and dynamic MutationObserver hooks to re-localize page updates.

Why

Ensuring global compatibility requires localization support in English, Hindi, Spanish, and French, matching the target demographic for the sustainability initiative.

How to test

1. Launch the application.
2. Open the language selector in the topbar header.
3. Select Spanish (ES) or French (FR).
4. Verify that the login screen, sidebar nav items, dashboard titles, and stats cards instantly translate and that the chosen language preference persists in LocalStorage upon reload.

Checklist

• Tested changes locally
• No lint/syntax errors introduced
• Matches all issue criteria

Files Affected

• index.html
• src/i18n.js

[knoxiboy]

@Shruti070107 Please review this pr and merge it

[knoxiboy]

@Shruti070107 Please review this pr and merge it