I have Office 2013 Professional Plus installed, English version with German language pack. As "Editing Languages" I have configured German and English, with German the default one.

Still, when I create a new e-mail and start to type, all the German words are highlighted because the proofing language is set to "English". Even when I create a new, empty e-mail and go directly to "Review", "Proofing Language", it is set to "English (UK)". 

Other users (using Office 2010) are reporting the same issue. How can this be changed?

Is this only occurs to Outlook (both 2010 and 2013) ? How about Word?

Check if the users are using custom Stationery, if so, remove the custom Stationery and check if the behaivor still persists.

Normally, we can find the custom Stationery under the following location:

C:\Users\%username%\AppData\Roaming\Microsoft\Stat

Thanks, I had a similiar idea. Unfortunately, the Stationery folder is empty.

The issue occurs only in Outlook 2010 and 2013. It does not appear in Word. Also it likely did not appear in Outlook 2007.