Local characters encoding problem

Having a very unpleasant problem with scrambled local characters in program 
call input parameters.
Maybe someone can advise me of how to solve it.
PC program makes a call to a AS/400 program passing some parameters as a 
unsigned char array, that is actually a packed structure. So it contains bytes, 
some of which are actually chars, some are numbers. PC has always one and the 
same local settings.
Probably my 3 AS/400 have different "regional settings". Result is that 
depending on the particular machine I must use 3 different manually built 
encoding tables.
We have only 8 local characters, so building this table itself is not a 
problem. But the problem is managing 3 different versions of the same program, 
that work on particular machine.
For exampl I have letter Ä (capital A with 2 dots above it) that comes to one 
machine as 0x0A and to another machine as 0x24.
So on one machine I must have a code row :

for (i = 0; i < 20; i++)
  if (Buffer[i] == 0x0A) ;
    Buffer[i] = 'Ä' ;

and on another machine :
for (i = 0; i < 20; i++)
  if (Buffer[i] == 0x24) ;
    Buffer[i] = 'Ä' ;

Stupid, isn't it?! Is there a chance to find some proper AS/400 parameter, may 
be some system variable or whatever, that can be tested in order to select 
proper comparison right side?

Should say here, that I understand, that probably correct way is to take care 
of this encoding is to push it to PC side, but this is not an option for 
certain reasons...

TIA,
Jevgeni Astanovski