When you say 'source code all websites', maybe you mean WordPress and LAMP websites? If so, ignore this comment. But source code of all cloudron packages and the final built images is all public - https://hub.docker.com/u/cloudron . you don't even need server credentials or install Cloudron for that matter to see the source code of apps.
For WP/Lamp, I consider the websites themselves as "data" and "configuration" and not "source code".
@zonzonzon I guess @PrimmaAnna is part of the Answer team. Basically, what was said - you have to submit translations upstream. Then, the Cloudron package will automatically pick up the changes.