This plugin provides a fallback locale next to a default locale.
The default locale is still used as the default when editing content, while the fallback locale is used as a fallback in the frontend if no matching locale was found.
To use a fallback locale go to Settings > Translate > Manage languages
Then edit the locale, that should become the fallback and activate the "Fallback" checkbox and save.
This plugin has not been reviewed yet.
Mar 25, 2021