As far as i can tell, You should be fine by using utf8_general_ci as of your database character-set and the same when you install MODx!
If you upgrade your MODx and have got question marks in return of multi-byte characters (like say Arabic) its because you have override the previous db connection set/method
Double-check the MODx config files to make sure that database settings are fine/ uf8
Change the database character configs by using php_myadmin & re-enter/edit the contents shown as question marks then save it!
??????)
