Plain text files - in most cases files with a txt extension - contain just textual information and offer no clearly defined way to inform the computer which language they contain. The most that OmegaT can do in such a case, is to assume that the text is written in the same language the computer itself uses. This is no problem for files encoded in Unicode using a 16 bit character encoding set. If the text is encoded in 8 bits, however, one can be faced with the following awkward situation: instead of displaying the text, for Japanese characters...

...the system will display it like this for instance:

The computer, running OmegaT, has Russian as the default language, and thus shows the characters in the Cyrillic alphabet and not in Kanji.
There are basically three ways to address this problem in OmegaT. They all involve the application of file filters in the Options menu.
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.
                     
- i.e. files with a .txt extension - : in
                                  the Text files section of the file
                                  filters dialog, change the Source File
                                        Encoding from <auto> to the encoding that
                                  corresponds to your source .txt file, for
                                  instance to .jp for the above example.
                     
for instance from .txt to
                                  .jp for Japanese plain texts: in the Text files section of the file filters
                                  dialog, add new Source Filename
                                        Pattern (*.jp for this example) and
                                  select the appropriate parameters for the source and target
                                  encoding
                     
OmegaT has by default the following short list available to make it easier for you to deal with some plain text files:
.txt files are automatically (<auto>)
                                interpreted by OmegaT as being encoded in
                                the computer's default encoding.
                     
You can check that yourself by selecting the item File Filters in the menu Options. For example, when you have a Czech text
                   file (very probably written in the ISO-8859-2 code) you just need to change the
                   extension .txt to .txt2 and
                   OmegaT will interpret its contents correctly.
                   And of course, if you wish to be on the safe side, consider converting
                   this kind of file to Unicode, i.e. to the .utf8 file
                   format.