feat(translations): add timezone support for en and ja locales (#1566)

This PR adds missing translations for the `timezone` validation tag in
both **English (en)** and **Japanese (ja)** locales.

Previously, the `timezone` tag did not have a localized error message in
these languages, causing it to fallback to the default tag name. This
enhancement ensures a better user experience for developers using these
locales.

## Changes
- Added `timezone` translation to  `translations/en/en.go`
- Added `timezone` translation to `translations/ja/ja.go`
- Added unit tests for `timezone` translation in
`translations/en/en_test.go`
- Added unit tests for `timezone` translation in
`translations/ja/ja_test.go`

**Make sure that you've checked the boxes below before you submit PR:**
- [x] Tests exist or have been written that cover this particular
change.

@go-playground/validator-maintainers