How to specify localizations for Flutter / .arb files?

The docs don’t seem to give any way for me to specify what languages my app supports:

Appstream provides translation information, so that software centers can inform users if the app is translated into their language. If the app uses Mozilla .xpi or Google .pak files for translation, the translation info is populated automatically. If the app uses gettext .mo or Qt .qm files, you’ll need to provide the prefix of these files with a <translation/> tag

My app is written with Dart & Flutter and the localization files are of type .arb. You can see my translation files here.

I couldn’t find any information on manually specifying which languages are supported, only auto-detected from the translation files. How do I specify my supported languages?