Hey there,
I am really sorry for the delay of this reply as it is a weekend for us here.
I've tested on your site, it seems like if i choose English language option (screenshot :
http://take.ms/GCF06 ) before login, then i approve the blog post from backend, system will send out English translation in the email.
If i didn't choose that language option before i login, the system will send out correctly with the German translation language in the email when i approve the blog post from backend, you can refer on my attached screenshot below.
By the current behaviour, the system have to check what language the current logged user chosen, then that email will respect it.
Do you know how your site moderator approve the blog post normally? e.g.
1. when the moderator receive an email someone published blog post under moderation, so this moderator view the blog post in front site, then approve it.
2. Or they will login from backend and approve it.
3. Or they will check on the frontend user dashboard approve those moderation post under moderate page.