Convert the current buffer from SERA to FIDEL.
The variable `ethio-primary-language' specifies the primary
language and `ethio-secondary-language' specifies the secondary.
If the 1st optional argument SECONDARY is non-nil, assume the
buffer begins with the secondary language; otherwise with the
If the 2nd optional argument FORCE is 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 &optional SECONDARY FORCE)