I installed Notepad++ 6.4.5 to Windows Server 2008 R2 in language: German. When i login in server with new user, i "forgot" my default GUI-Localization-Language. The Program was english, instead of german. Means that the Localization in Settings -> Preferences was empty/didnt have a default language set. i had to manually choose German.
Is this a bug?
When you make a configuration change of any kind (colours, edit preferences, localisation, ...), does it stick?
If not, then you have a permission problem on the configuration files.
If yes, then it could be that the installer didn't have the proper permissions.
Are you using %APPDATA% ?