DE language support for the plugin

[DavidQinqianWu]

#670
This change allows the Navigation plugin to switch to German (DE) based on the Strapi system language. I’m not sure whether my implementation fully meets your contribution guidelines. If there are specific rules or conventions I should follow, please let me know.

[CodeVoyager]

Thank you, for your contribution. We'll have a look and give feedback (hopefully) soon.