Problems with NLS_LANG (Oracle Reports 6i)

I should use russian characters in report I made (it has 3 language choice : serbian, english and russian) , so I used CROATIAN_CROATIA.UTF8 for NLS_LANG (everything works ok on my machine with that settings). But on the other machines (with users) NLS_LANG is set as AMERICAN_AMERICA.EE8MSWIN1250 and they didn't see correct results even for serbian characters .There are similar reports which use AMERICAN_AMERICA.EE8MSWIN1250 and I use this NLS_LANG setting(AMERICAN_AMERICA.EE8MSWIN1250 ) when I produced them on my machine and users can see everithing properly. But now, when I need russian characters, I changed NLS_LANG in registry and there is a problem by users. I know that users who wants to see russian characters should use NLS_LANG as CROATIAN_CROATIA.UTF8 , but user who wants to see serbian characters use AMERICAN_AMERICA.EE8MSWIN1250 and they don't need russian characters, but they can't see serbian characters properly.
 So here is my question: Which NLS_LANG setting should I use when I compile report for both purposes (for users with NLS_LANG as AMERICAN.... who will use only serbian characters and forusers with NLS_LANG as CROATIAN..... who will use russian characters)? I want to have only one version of that report (not two difrent compiled versions). Should I use other setting (I can only change setting for russian characters and users who used AMERICAN.... mustn't change NLS_LANG!). Is there some other way to solve problem (some EXE file which can change registry settigs)? I read about ALTER SESSION command, but it works only for language and theritory, not for character set in NLS_LANG. Thanks, anyway!!!

                                                Nenad