FEAT: Add unit tests for i18n/validator.go

[andrinoff]

Is your feature request related to a problem?

i18n/validator.go has validation logic for translation completeness but no dedicated tests. Missing or malformed translations could go undetected.

Describe the solution you'd like

Add i18n/validator_test.go with tests for:

• Detection of missing translation keys
• Detection of mismatched plural forms
• Validation report formatting
• Edge cases (empty language, empty key map)