I just noticed an error in the german language file plg_jem_mailer.ini which causes the file not to be loaded:
on line 200
PLG_JEM_MAILER_LOG_SEND_ERROR="Fehler beim Senden einer Emails an "_QQ_"%s"_QQ_"_QQ_
at the end "_QQ_ has to be removed in order to work (compared to the english version)
This bug may be due to strange Transifex behavior
Discussed here: issues.joomla.org/tracker/joomla-cms/15378
This message is used only in logging as far as i can see in the code.
You have turn on Joomla Debugging (see here docs.joomla.org/Using_JLog) and you have to set the debug plugin to log almost everything. Then change an email adress to a wrong adress and book an event this user.
I have used your PLG_JEM_MAILER_LOG_SEND_ERROR in my german language file and it worked
from the log file:
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