Hi Andy,
The language constant reported is indeed missing from the language file. Can you apply the following line in EasySocial's language file and see if the language strings generates correctly now in a new notification email?

APP_EVENT_PHOTOS_EMAILS_COMMENT_PROFILE_PICTURE_ITEM_SUBJECT="{actor} posted a comment on your photo"

If this solves your issue, do let us know so we can add this fix into the next release, thanks.