I found a solution myself.
It works, when I use the vim editor with the autocommands
: autocmd BufNewFile,BufRead *.abc setf abc
: autocmd BufNewFile,BufRead,BufEnter *.abc set encoding=latin1
: autocmd BufNewFile,BufRead,BufEnter *.abc set fileencoding=latin1
In this case I have to keep the $LANG environment variable (utf8 with me),
it was not good to change it to C or latin1. With the other text editors I know I did not find a solution.
This is an abc group, so I won't discuss the topic further.
ABC works, and I'm happy with the solution.