Add multilanguage support with per-language rejection phrases

[antoinekm]

Hey @hotheadhacker,
This PR adds support for multiple languages to the api.

Changes

• Restructuring the project to use a directory-based approach for language files
• Moving existing English reasons to reasons/en.json
• Adding support for the Accept-Language HTTP header
• Implementing language detection and fallback logic
• Adding initial support for German (de) and French (fr) translations
• Updating documentation to include multilanguage usage

How to Test

• Make a request with Accept-Language: de to get German rejections
• Make a request with no language header or an unsupported language code, and verify it falls back to English

Related Issues/PR
Closes: #3, #5

Compared to PR #5, this solution offers:

• Cleaner file organization: Each language lives in its own file, making it easy to see what's available at a glance
• Easier contributions: Want to add Spanish? Just create es.json instead of navigating a huge JSON file
• Fewer git conflicts: When two people work on different languages, their changes won't collide
• Better memory usage: We only load the language file you need, not every phrase in every language
• Industry-standard approach: Uses the same patterns as other multilingual projects
• Simpler code: No need for complex shuffling logic or global state management

[andreulhoa]

I would like to contribute with pt-BR nopes phrases following Add multilanguage support with per-language rejection phrases #17 's implementation