Rework the multilingual page

[jmooring]

https://gohugo.io/content-management/multilingual/#missing-translations

If a string does not have a translation for the current language, Hugo will use the value from the default language.

No, it will not. For example, if french is the default language, and a translation is defined in fr.toml but not in en.toml, then nothing will be shown on the english page. However, if french is the default language, and a translation is defined in en.toml but not in fr.toml, the english translation will be shown on the french page.

If no default value is set, an empty string will be shown.

I'm not sure how one would set a default value, and empty strings are, by definition, never shown.

[LHSnow]

Also, if the language is 'en' in the hugo config, a translation file named en_US.toml in /i18n/ does not get considered. A file named en.toml does, however, so the only example that is correct regarding file naming in the docs is fr.toml. It does not seem to matter if the language is instead named 'en_US' in the config, either.

[jmooring]

Closing in favor of #2099.