Add Multi-language Translation Support

[valerius21]

Application Translation Support

To make seal3d accessible to a broader audience.

Scope

• Prepare and externalize strings for translation (i18n)
• Add support for at least one additional language (e.g. German)
• Implement a method for easily switching languages (UI selector with persistance)
• Document how to add further translations

I don't want to introduce a bloated library. open for suggestions.