## Summary
- Update README with the list of supported languages for EPUB files.
- Update USER_GUIDE with an extended list of supported and unsupported
languages.
## Additional Context
For weeks, I thought this firmware only supported English, because I
remember you saying that full language support would only be possible
after implementing proper font rendering. I also remember mentioning a
separate Korean fork, Vietnamese issues and so on.
All of this made it clear that this system doesn't support my languages.
I was surprised when I saw a Reddit post with a photo of a book in my
native language. Only then I did learn that such languages are
supported. Therefore, mentioning the supported languages would help
future buyers and new users.
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### AI Usage
Did you use AI tools to help write this code? _**NO**_
Yaroslav