I am using OmegaT version 2.5.5 and Windosw Vista.
Unfortunately I have read all the contents of help file.
I found nothing special there.
It was too brief.
Lack of details.
The OmegaT manual wrote:
Chapter 11. Working with plain text
2. The OmegaT solution
Change the encoding of your files to Unicode
open your source file in a text editor that correctly interprets its encoding and save the file in "UTF-8" encoding. Change the file extension from .txt to .utf8. OmegaT will automatically interpret the file as a UTF-8 file. This is the most common-sense alternative, sparing you problems in the long run.
Then, I changed that file encoding to ANSI.
I got not a problem at all in both languages.
If you apply the above part from the manual, it should solve your problem.
The other solution (if you want all .txt files to be interpreted as UTF-8), is to change the default encoding for the .txt extension in Options > Files Filters > Text Files.