I used this method which is probably bad practice, but I couldn't find another solution.
With your file manager/FTP go to core language file: Example is for the installed German language
Open in a utf-8 reader.
You will be able to see a list of all English strings and their German counterparts: Example
Add your labels (strings) and their translations and save.
[-all good but this library must now be reread in-]
Using your web interface go to www.mysite.com/admin/config/regional/language
Delete the language you are editing. This will clear the DB of the translations but the edited file will remain untouched.
Now select "Add Language" and select your language again. The system will now read-in your updated list.