Fluent

The [fluent] table is optional but allows modifying the translation parameters if necessary; we recommend using the default values.

fallback Identifier of the fallback language (optional)
shared The name of the file used for shared messages (optional, default core.ftl)

The default fallback language is the primary language for the project defined by the top-level lang setting.

The shared file must exist in the locales directory, ie: site/locales/core.ftl.