> the translation is deleted without any trash process (what could be acceptable but not consistent with eZ experience)
Then seems we might have overlooked something during your requests for this feature. Trash of translation was not a feature which existed in eZ Publish before so that was never part of the intended scope here. It will have to involve a larger change to how trash works as it is content (including all translations) centric today. So any such change would be a somewhat major change to logic and API's, something we would need to request from our product team for considering placing on the roadmap for future releases.
> an error is displayed "Cannot delete main translation from content." (what is not consistent in terms of content management)
This sounds like something we should improve on. For instance disabling the button on main translations with help text making editor aware they can not delete main translation, and thus would need to change main translation. Or simply not exposing the button. This is already the case on v2, and we thus should reflect the same logic on v1.13. Being able to change main translation is a feature I know will most likely be added in v2 in a future release to be able to delete the original langue of a content item (main).
Last part on changing main language I can't really promise for v1.13 as dedicated translation tab was added first in v2.0 and thus not available as a feature in 1.x. However maybe this is not the most pressing need for you yet given your original use cases seemed to imply you mainly needed to delete additional translations which might contain mistakes, as opposed to removing the original source of the content which often is solved by deleting or hiding the content itself.
Regardless of which of these features / changes you would like to request, may I ask that you move this discussion to email thread with email@example.com. As this public issue has been technically solved and shipped as part of a release already it will soon be closed, so not the most optimal place for the discussion on followup features / improvements / issues.
VP Customer Success