I've said several times "there is always a user." I used this slide in my 2017 Tridion Developer Summit presentation.
The Translation Manager (TM) service is responsible for seeing if there are translation jobs to send as well as seeing if there are items to retrieve from the language system (TMS or Worldserver). It is an automated user that uses the Translation Manager configuration settings to know which system to check and how frequently to check for updates on the content or language side.
We looked closer at the TM service in one setup that had network connectivity issues between SDL Tridion and SDL WorldServer.
The final issue was likely a network policy issue that blocked or otherwise changed the traffic between the two systems.
To realize this we logged on as a different user and ran the Translation Manager (TM) service in a command prompt, as a quick and easy way to temporarily see what's going on. Note that closing the command prompt will stop this instance of Translation Manager.
> "TranslationManagerService.exe /d"
At this point the logs showed differences between this user and the default Network Service impersonation user.
Though the above quick check technically works, closing the command prompt, or restarting the machine will of course stop the TM service.
Set a slightly more permanent user by changing the MMC Snap-in settings and running the service under this account.
But this is also a work around.
If you detect network issues, do fix the root cause. Running TM from a command prompt and the MMC Snap-in on the server are quick work arounds.
If you can wrap your head our system users and services, read this interesting post from Likhan.
Have a favorite "it's just a (system) user" story to share? Comments appreciated.