If other languages are not defined show english
You could choose which value to translate your content into using the |t($lang) filter based on some condition. You condition could be a lightswitch field on each entry saying whether it's been translated or not, or check if a content field is empty or not.

So something like

set lang = entry.lang

if entry.isTranslated == false {
   set lang = 'english'
}

{{ content|t(lang) }}