Convert the characters in region from SERA to FIDEL.
The variable `ethio-primary-language' specifies the primary
language and `ethio-secondary-language' specifies the secondary.
If the 3rd argument SECONDARY is given and non-nil, assume the
region begins with the secondary language; otherwise with the
primary language.
If the 4th argument FORCE is given and non-nil, perform
conversion even if the buffer is read-only.
See also the descriptions of the variables
`ethio-use-colon-for-colon' and `ethio-use-three-dot-question'.
(fn BEGIN END &optional SECONDARY FORCE)