Multi-language support

[eretl]

Description

Adds multi-language support to the web interface. Every user-visible string in the admin panel and the public pages is now looked up from a translation file instead of being hardcoded.

Added Czech translation.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor / Chore

Technical Details

• Database changes: No
• Storage backend affected: No
• Usage of AI: Change was written with Claude AI assistance, mainly the Go package

How Has This Been Tested?

• Manual Testing: Tested the main functionalities - Upload, Delete, Download, Visually checked for missing translations
• Environment: Docker inside WSL2

Checklist

• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.