Feat: added i18n example (with dynamic translation)

[mrbuslov]

Description

This PR introduces an example of dynamic language switching using the aiogram i18n utilities. By default, aiogram handlers store the language context at startup, preventing runtime changes. This PR provides a workaround by leveraging i18n.context() and i18n.use_locale(new_lang) inside the /language command handler, allowing messages to be dynamically translated during execution.

Additionally, a BotI18nMiddleware class has been implemented to manage locale retrieval dynamically, enabling further customization (e.g., fetching the locale from a database).

Fixes # (issue)

Type of change

• New feature (non-breaking change which adds functionality)
• Documentation (code examples or any documentation update)

How Has This Been Tested?

This feature has been manually tested by sending /start and /language <new_lang> commands in a bot instance. The output messages correctly reflect the language change in real-time.

• Tested with different language codes (en, ru, uk) to ensure proper translation.
• Verified that the middleware correctly sets the default locale.

Test Configuration:

• Operating System: Linux
• Python version: 3.11.6

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

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[mrbuslov]

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