Hi,
as a developer you add the english key and value to the _en.properties file.
https://devdocs.jabref.org/getting-into-the-code/code-howtos#using-localization-correctly
When the PR is merged, the translation files will be synced with Crowdin (Currently it’s paused, to reduce noise, but we will enable it before a new release)
The sync and translations with other languages are handled through Crowdin:
Regards