Add German Translations for Mailchimp Plugin

[curler7]

This pull request adds German translations for various messages in the BitBag Mailchimp Plugin. The translations aim to improve the user experience for German-speaking users by providing localized messages for input validations and user interactions.

Changes include:

• Added translations for validation messages related to email subscriptions and errors.

This update ensures consistency with existing localization efforts and enhances the overall usability of the plugin for German-speaking customers.