Multilingual Enhancement Assistant for Existing Website

<context>
Current setup:
- Single-page website (index.html)
- Modular CSS theming system with separate CSS files
- Functional JavaScript implementation
- Stable production environment requiring careful modifications

Target:
- Add multilingual support starting with German (base) and English
- Maintain existing theming system
- Ensure scalability for additional languages
- Preserve current functionality

<instructions>
Provide solutions that:
- Minimize changes to existing structure
- Maintain current CSS theming capabilities
- Follow progressive enhancement principles
- Preserve site performance
- Consider SEO implications
- Keep code maintainable

<output_format>
Structure responses as follows:
1. Impact assessment on existing system
2. Step-by-step implementation plan
3. Code modifications with detailed comments
4. Testing guidelines
5. Documentation requirements

<technical_constraints>
- Must work with separate CSS files
- Avoid breaking existing theming system
- Maintain current performance levels
- Support offline functionality
- Allow easy addition of future languages
- Ensure backward compatibility

<implementation_requirements>
Code solutions must:
- Clearly separate content from functionality
- Maintain existing CSS variable system
- Include fallback mechanisms
- Be SEO-friendly
- Support browser compatibility

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Verbesserungen:
ich denke du solltest zu erst alle css dateien ansehen und dann die änderung nocheinmal überdenken.
- Die Buttons für die Sprache sind visuell für den nutzer nicht zu sehen da der header verdeckt.
- Bitte gleich auch den Header korrigieren. Der Header sollte beim Scrollen stehen bleiben und nicht verschwinden.
- In der Mobileansicht wird die Sprachauswahl nicht angezeigt.


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Die Translation Buttons sind