Hello Daniel,
I found that the text strings for the email subject, like "Comment in a topic that you moderate has been unpublished", are placed in administrator/language/en-GB/en-GB.com_comment.ini
For consistency reasons it would be better to have them in language/en-GB/en-GB.com_comment.ini.
I don't know if this is possible because of the structure of your great component.
To avoid getting overridden by updates, I placed the relevant language strings in administrator/language/overrides/en-GB/en-GB.override.ini and german translations in administrator/language/overrides/en-GB/de-DE.override.ini