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 a particular user has. 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.