Replace all the FIDEL characters in the region to the SERA format.
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, convert
the region so that it begins with the secondary language; otherwise with
the primary language.
If the 4th argument FORCE is given and 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 BEGIN END &optional SECONDARY FORCE)