almost complete i18n + other fixes

Almost complete i18n. I just didn't touch the auth module yet. Maybe later.

I removed lots of no longer used translations. Except for tag functionality, which seems to work only if some code was uncommented)

I also made the HTML valid - there wasn't even `<html>`. ;-) Guess you wanted to enforce quirks mode because without it `textarea` was a bit too large, but the only thing that was needed was `box-sizing: border-box`.

Sorry for lots of commits. Wklej doesn't build on my localhost, so I need to push the changes to the production every time I change something.

You can test the changes live at https://wklej.stratushost.eu/.