Just for the record, this is exactly what Weblate should be able to do - fetch the data from upstream JSON, let translators translate the strings via an AMOS-like UI, generate the required files and submit them via a pull request in the upstream Github repository.
We were considering to use the current AMOS but
- there would be a non-trivial custom development needed to keep the upstream translations with our own ones in AMOS in sync
- there is an ongoing project to switch over to Weblate
so it did not sound like a wise approach for now.
We still want to provide a common environment for our translators community for sure.