Running windows 2008 r2 adprep on a server in a different language

When you have a older active directory system, for example based on a Windows 2003 or Windows 2008 server, you have to upgrade the ads schema by running adprep.exe

The adprep.exe tool is found on the windwos 2008r2 DVD in the support\adprep directory.
You can copy over the whole content of the directory (including subdirectories) to your ads master  server.

If the "old" server is running a 32-bit windows, then you can start adprep32.exe instead of adprep.exe.

Now adprep.exe should tell you something about "At least windows 2000 sp4 required"
If you don't get any messages from adprep, then the languages of the windows 2008r2 dvd and your old ads server do not match.

It might be tempting to just type "c" for continue, it will then do some of the work, but unfortunally not everything you need.
adprep does write a logfile, which can be found in C:\windows\debug\adprep (or C:\windows\system32\debug\adprep for a 32-bit system)
You will see error like

There is a syntax error in the input file
Failed on line 169.  The last token starts with 's'.
An error has occurred in the program

The reason for this is, that there are some strings missing for the adprep utility.
To get it working correctly, you have to rename the language subdirectory to match the servers language. (Chaning it in the control panel is not enough)
For example if your new server is running german, then the subdirectory is called de-de.
If the old server for example is installed in english, you have to rename the de-de directory to en-us.

Now you can again run adprep, you will receive the prompts in the original language from the dvd, but everything will process as expected.