Programming

Perhaps, the "fallback to English translations file at runtime" is obscuring errors.
Might be worth redefining the system to throw an error when a translation key in the chosen language isn't found. Even if that's only done in Dev, whereas the fallback happens in prod.
This will ensure a translation file has all the keys, even if the values are still default.

Some tooling for you to easily add a new key, and have it also add that to all language files as [word] or something. So, the English word is still used, but the square brackets shows that it's untranslated.
Maybe some tooling to find all values that have the [], to generate a translation to-do list.
Probably a tool to create a new translation file as well, which would duplicate the English file, but apply the "this is not translated" pattern to all the values

Quite a lot of IDEs will key you just click "add missing properties" action on the translation object to create a language file.

Obviously I don't know your codebase but couldn't you do something like the following?

function loadTranslations(locale) {
  const fallbackTranslations = require("/i18n/en.json");
  const translations = require(`/i18n/${locale}.json`);
  return {
     ...fallbackTranslations,
    ...translations
  };
}