Set translation author to committer when importing translations pushed to VCS directly by localizers.
A couple of thoughts: We also have to remember that single contributor can map into multiple emails in VCS. Those VCS-emails would require additional confirmation (e.g. via verification email) if contributors would add them to their accounts.
If we find a match between the commit author and Pontoon users, we're good. If we don't, we can create a (fake) user account. I wouldn't worry about contributors who use multiple email addresses at this stage. This is something we can work on as a followup.