How to convert *text* to {\i text} with sed

see https://stackoverflow.com/questions/5858200/sed-replace-every-nth-occurrence

The solution uses awk rather than sed, but "use the right tool for the job". It may or may not be possible to do in sed but, even if it is, it will be a lot easier in a tool like awk or perl.

If you are okay with using perl

$ s='abcTestString : "abc {ccb} bbc",'
$ echo "$s" | perl -Mopen=locale -Mutf8 -F: -lane '
               $F[-1]=~s/\{[^{}]+\}(*SKIP)(*F)|[a-z]+/$&=~tr|abc|абц|r/ge;
               print join ":",@F'
abcTestString : "абц {ccb} ббц",

• -Mopen=locale -Mutf8 unicode settings (thanks to this wonderful answer tr analog for unicode characters?)
• -F: -lane use : as field separator, saved in @F array (See https://perldoc.perl.org/perlrun.html#Command-Switches for other options)
• $F[-1] last field of @F array
• \{[^{}]+\}(*SKIP)(*F)|[a-z]+ here we say that [a-z]+ portion has to match but \{[^{}]+\} should be left as it is
• $&=~tr|abc|абц|r perform transliteration for the matched portion
• ge the g modifier for replacing all matches, e modifier to allow the Perl code in replacement section

If this is too big a code to handle from command line, change it to a program

$ echo "$s" | perl -MO=Deparse -Mopen=locale -Mutf8 -F: -lane '
               $F[-1]=~s/\{[^{}]+\}(*SKIP)(*F)|[a-z]+/$&=~tr|abc|абц|r/ge;
               print join ":",@F'
BEGIN { $/ = "\n"; $\ = "\n"; }
use open (split(/,/, 'locale', 0));
use utf8;
LINE: while (defined($_ = <ARGV>)) {
    chomp $_;
    our @F = split(/:/, $_, 0);
    $F[-1] =~ s[\{[^{}]+\}(*SKIP)(*F)|[a-z]+][use utf8 ();
    $& =~ tr/abc/\x{430}\x{431}\x{446}/r;]eg;
    print join(':', @F);
}