sort data alphabeticlaly with more languages

Hello, I am sorting data on my iOS app by a field in database. See query below in project. But people who use other language, namely German, I want to sort too by another field in db. How can I do that by adapting my php query?

$sql = "SELECT *
FROM SchifferProducts WHERE Instock = 'true' ORDER BY MainCat desc, KwalOmschr asc ";