Resolve user locale by checking the Telegram language code as-is, then its Babel-normalized form, then the base language. This fixes lowercase regional codes such as pt-br failing to match available translations like pt_BR. Add tests covering region-code variants and fallback behavior, and update i18n documentation plus changelog notes.

Description

This PR improves locale detection in SimpleI18nMiddleware by resolving the user locale in this order: Telegram language code as received, Babel-normalized locale, then base language. This fixes cases where Telegram returns lowercase regional codes such as pt-br and translations exist under pt_BR, which previously caused fallback to the default locale.

The change also adds test coverage for region-code variants and fallback behavior, updates the i18n documentation to explain this resolution flow, and includes a changelog note.

Fixes #1755

Type of change

• Documentation (typos, code examples or any documentation update)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Added and updated unit tests for locale resolution behavior, including:
  • Direct match for Telegram locale code
  • Babel-normalized match for region codes
  • Preference when both raw and normalized locale keys exist
  • Base-language fallback for regional codes
• Verified behavior for examples such as pt-br to pt_BR and en-US to en

Test Configuration:

• Operating System: Linux 6.19.3-2-cachyos
• Python version: 3.13.11

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes