When you export a language from your localization project, the empty translations you have in that language can be filled in in the exported file with translations from another language in the project - the fallback language.

There are multiple ways to set a fallback language.

How to set a project-level fallback language

The owner and administrators of a localization project can set a general fallback language in the project's settings.

This language applies to all language exports, unless an individual fallback language is chosen.

With the API, the project-level fallback language can be changed with this method with the fallback_language argument.

How to set individual fallback languages

In the language page, in Export > Advanced Options, you can choose a fallback language only for the language you are exporting, without affecting exports for the other project languages. This settings is for your user only and it overrides the project-level setting, if one is set.

You can choose as fallback language any of the language in the project to which you have access.

If you're using POEditor code hosting service integrations, you can also set individual fallback languages for any linked language, by clicking on Settings > Add export rule.

To get the same result with the API, you can use this method with the argument fallback_language.