[FE] Dashboard Multi-Language Switcher (i18n) #428

Description

Extend the i18n localization currently scoped to the checkout link to the entirety of the merchant dashboard UI.

Requirements and context

• Allow changing locale via a header selector.
• Provide ES, PT, FR translations for core pages.

Suggested execution

• Scale next-intl configuration and wrap all hardcoded English strings in hooks.

Example commit message

feat: support multi-language localizations across entire dashboard

Guidelines

• Complexity: Medium (150 points)