*OmegaT tip*
Name the target file by appending a language code,
... e.g. if my source file is file.txt and my project is en => es, I press Ctrl+D and the file created in the target folder is file_es.txt:
Options > File filters
Choose the file family and Edit. That will open a window. There, in the target filename column, use this:
${nameOnly}_${targetLanguageCode}.${extension}
Then, press OK, OK and OK, in every window and restart the application.