More specifically, the error is because the package has never been added to Packagist. The only reference on this forum seems to date back to 2018 https://discuss.flarum.org/d/3216-persian-farsi-language-pack/14
I just checked as well, the extension won't work past beta 7, so work would be needed to update it anyway.
Generally, extensions that don't have a discussion under the Extension tag here never reached a "ready to use" state.
