Description

Add source_locale parameter to aiogram.utils.i18n.I18n and handle it in aiogram.utils.i18n.middleware.SimpleI18nMiddleware.

When source_locale is set (defaults to default_locale), and the user's language matches it, gettext() returns the original untranslated text even if the source locale is not compiled into a .mo file.

Previously, a user with language_code matching the source language (e.g. "en") would receive the default_locale text instead of the original, because the middleware fell back to default_locale when the source locale was not found in compiled locales.

Fixes #733.

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?

• tests/test_utils/test_i18n.py — added assertions for source_locale attribute, added test_init_source_locale, added test_gettext_source_locale, added test_source_locale_as_user_language
• All existing i18n tests pass (40/40)

Test Configuration:

• Operating System: Linux 7.0.10-2-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