Replace all the FIDEL characters in the current buffer to the SERA format.
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, try to convert the
region so that it begins with the secondary language; otherwise with the
If the 2nd optional argument FORCE is non-nil, convert even if the
buffer is read-only.
See also the descriptions of the variables
`ethio-quote-vowel-always' and `ethio-numeric-reduction'. (fn &optional SECONDARY FORCE)