Modern DXF/DWG files typically save texts in the Unicode-8 text encoding, which is what HighDesign assumes as the input encoding during the conversion. Older files, up to version R14, would use plain ASCII, but we've encountered files saved in the Windows ISO encoding.
Unfortunately, DXF/DWG files can present themselves in a number of variations. The variables are the age of the file, the software that produced it and the part of the world where it came from. I'd suggest you send the file to the Ilexsoft support email, so we can inspect it.