fix: multilingual issues in feeds and 'nav.html'

[Smtbook]

Feeds in other language seemed broken, and I changed 'nav.html' to fix issues when the length of language code is not 2.

This is my first PR, warn me if I did anything wrong.

[welpo]

Thanks for your contribution!

I will translate the text from the new feed_style files and ensure the default (en.xsl) is loaded if there's no language-specific file. This way we avoid errors in feeds for users who use a different language but haven't created the feed_style/{lang} file.

[welpo]

All done! Merging :)

Thanks again.

EDIT: Accidentally closed it. Oops.