On a Samsung Galaxy device, open the Settings app. Scroll down to General management and tap on Language and input.
If you don’t see the Language and input option in General management, then your device doesn’t have this feature. You can download an app that makes adding languages more accessible, such as Google Translate (ongoing update). Tap on Add language and choose the language you want to add from the Languages table. If you want to set the new language as default, check the box next to Default language.