In my opinion, if the translation is totally wrong of poor in quality, I would go ahead and change it. You don't need to consult the author of a poor translation. But you might want to contact him/her so that he/she doesn't keep making the same errors.
If the change is more a matter of taste (e.g. formal language vs. colloquial language), I think it's better to ask in this forum first so that everybody is in the same page and to make sure that the translation is consistent everywhere. If nobody answers after some days, I guess you can consider yourself the "owner of the translation" and do as it pleases you most.
Sometimes, I write "Comments" justifying my choice of translation (often, because I have bothered enough to check the real context of the phrase...) in case someone might feel the temptation to change my translation without giving it enough thought.
About the bug in updating the author of a translation when it is changed, I reported it long ago ( viewtopic.php?f=8&t=6869&p=26859#p26859
). I guess you can take that information as the "author of the original translation". Not very useful, but that is what it is today.