A step forward in the internationalization of very common templates
Having this at translatewiki might serve better than having this at commons. For new/small projects, it can be tough to understand/implement wikisyntax. Being able to translate templates without getting into the wikisyntax would be helpful for such projects. However, starting along this path may make translatewiki more of a GUI editor for templates than internationalization. Since a GUI editor for mediawiki is already under development, this might be a redundant step (even though, as I said, more useful than at commons).
I think you may be missing my point: I stated: Not going to happen.
I ask you to reconsider the idea, at least for Wikimedia as per this reply from SPQRobin and others. It's not only for commons. I think it'd be a useful move. See also bug #35332 for issues regarding your proposal with the translation extension. Regards.
I feel that translation of these templates would be more suitable to happen somewhere inside WMF, maybe meta or commons?
And would it be possible then to use the message everywhere? - We're just asking to create the text (such as "Done") and translate it on as much languages as possible. Regards.
Perhaps a first step is to draw a list of the words to be translated; such a list could be inspired from (a subset of) the Swadesh lists and from various translations on Meta and Commons.
Perhaps also it is a good idea of creating an extension similar to "Wikimedia messages" with very common words (called "CommonWords"?) with frequent words and an extensive qqq explanation to clearly define each word and avoid mistranslations.