Hi, I don't think that's right, sorry. It refers to the user's own account(s) in various MediaWiki projects. See w:Special:ListGroupRights for an example usage of that message. This page is a list of rights that users can have. As you can see, one of those rights is the right to merge their account in the various MediaWiki projects into one unified account. See meta:Help:Unified login for background information on this system.
In this message, "their" is a case of the "singular they", so it should be translated as "merge his/her account". In Interlingua I translated as "fusionar su conto" but perhaps "fusionar le proprie contos" would be better.
Oh, now I understand! I thought it was referred to an administrative right about merging someone else's account. I have corrected it to "Fusionar a conta propia" rather than "Fusionar a súa conta" because I find it too ambiguous. Thanks for reporting.
By the way, I use Interlingua as one of my assistant languages due to its similarity with Galician.