$ export LANG="en_US.UTF-8"
$ echo 'aöæaæaæa' | tool -utf8 'æa'
2
4
6

$ export LANG="en_US.UTF-8"
$ echo 'aöæaæaæa' | tool 'æa'
3
6
9

echo 'aöæææ' | awk '
{
  split($0, chars, "")
  for (i=1; i <= length($0); i++) {
    if ( chars[i] == "æ") print i-1
  }
}'

charLoc() {
  [ "$#" -lt 2 ] && { printf 'insufficient argumentsn' >&2 ; return 1;  }

  printf '%s' "$1" | awk -v arg="$2" '
  {
    split($0, chars, "")
    for (i=1; i <= length($0); i++) {
      if ( chars[i] == arg) print i-1
    }
  }'
}

charLoc 'aöæææ' "æ"
2
3
4