At run-time and in the latest .dwg versions, text is stored in Unicode. Older versions store text as MBCS. To convert text to Unicode it is necessary to know a code page, which is stored in the .dwg/.dxf file. But very old versions (or .dxf files generated by some applications) may have no code page information. In this case a default system code page is used. Implementation of the systemCodePage can be found in the Kernel/Extensions/ExServices/ExSystemServices.cpp file.
Loading such a file can be different depending on the computer's language settings. On non-Windows platforms, the OdCharMapper should be initialized with the adinit.dat file containing text conversion tables for MBCS code pages. The file resides in the /Kernel folder. Also, see the Drawing/Examples/OdReadEx/OdReadEx.cpp file.