Describe the bug

In i18n/validator.go:127-139, report string is built with repeated +=:

report += fmt.Sprintf("  [%s] %s: %s\n", err.Language, err.Key, err.Message)
report += fmt.Sprintf("  [%s] %d missing keys:\n", lang, len(keys))

For large validation reports with many languages and keys, this creates O(n²) allocations.

To reproduce

Run i18n validation with many languages and missing keys.

Expected behavior

Use strings.Builder instead of string concatenation.