I have this problem in itwiki: I wanted to translate the page Le Transperceneige from enwiki, but another user already started the translation. The problem is that that user's last edit was on Jan 2, 2016 and now I can't start the translation of the page.
Topic on Talk:Content translation
Hi @Fringio. Thanks for your feedback.
This is a current limitation of the tool that can be problematic in some cases. This ticket captures the issue and a proposal for fixing it that goes along the lines of your suggestion. You can subscribe to the ticket to track the progress, or add any comment. There is also a proposalto automatically discard very old translations, that can also help to alleviate the problem of articles being blocked.
As far as I know there are not immediate plans to work in this area, but I linked your comment from the corresponding ticket for the team to be aware of it. So the only workaround I can think of is to copy the source article as a subpage of your user page and translate it.
Thank you for the response. I've subscribed to the ticket.