The language is stored in the user specific part of the registry (file
NTUSER.DAT in the user directory).
When the user is logged in, you can access the user settings at
HKEY_USERS\<SID>
using
regedit or the Windows API registry functions called from a program with administrative privileges. When using a program, the users must be enumerated first to get their SIDs.
When the user is not logged in, the registry can be edited offline using the
Reg[
^] utility. See for example
Modify the Registry of Another User[
^].
While this is not a C# solution, it should be no problem to execute the commands or write them to a shell script that can then be xecuted.