Only render languages which are 100% translated in the "Website" project

There is still ongoing discussion about whether to refine this further in #78 (closed), but this at least eliminates the languages for which we can't even render the menu items properly.

It also updates the documentation to explain that you can avoid this behaviour by running ./tools/update_langs.py --partial instead of ./tools/update_langs.py. This will allow you to render partial translations again.