(Bug 60336) Fix sortkey for multibyte characters

- Use mb_substr( $this->rowSortkey, 0, 1 ) instead of $sortKey{0}
- Move some stuff around to be more expressive

Did an eyeball check because writing a test for the monster needs to be
done at another time.

[1] https://bugzilla.wikimedia.org/show_bug.cgi?id=60336