I'm in a similar boat. I just brought an unlocked (formerly AT&T) G900A from USA to Turkey and want to change the system language to Turkish. The default system languages didn't include Turkish so I followed a guide (https://udhyantimilsina.com/2013/08...o-set-multiple-languages-on-android-os-4-2-2/) to use MoreLocale 2 to change it to Turkish, but unfortunately it only half-worked. All the app names are now in Turkish, but other things like every option in Settings is still in English.
It looks like the G900A running Android 5.1.1 is not rootable, so I can't use another app like Language Enabler (https://play.google.com/store/apps/details?id=com.wanam) which requires root. Is there anything else I can do to fully change the system language to Turkish?