You can able to change Benutzerkont to Benutzerkonto from translate.csv file from relevant language pack folder
 
Folder Path : 
app/design/frontend/<package>/<theme>/locale/languagecode_COUNTRYCODE
and put your translated strings inside this translate.csv file.
 
Please check following steps:
1) German_LocalePack  folder path (Please create following translate.csv file with following folder path): 
app/design/frontend/<custom package>/<custom theme>/locale/de_DE/translate.csv 
 
Note : If folder is already exist, you no need to create that folder
 
2) Right click and open translate.csv from notepad or notepad++
3) Add following code :: "Benutzerkont","Benutzerkonto"
4) Save  the file
5) Flush the cache and check.
 
 
Note: 
Here are some tips and tricks:
1. All strings should start and end with double quotes and should be separated with comma (,), not a semicolon or any other sign
"Benutzerkont","Benutzerkonto"
 
Refer the following links:
http://blog.belvg.com/editing-magento-translate-csv-file.html
https://tomrobertshaw.net/2010/03/magento-themes-using-locales-with-translate-csv/