In Samsung Galaxy S5 (also other devices) if you lost your language and want to set it back to your language or you want to set to another language, then you have to go to the Language setting menu.
If you don't understand the existing language, you can do by seeing the icon of each step as I will show below.
To go to the language setting menu in Samsung Galaxy S5 (also for other devices using Android 4.4/ Kitkat) :
1. From main display, choose and tap/click the Apps icon
2. Then tap the Setting icon
3. Choose the 3rd row, that is the Sound and Display menu, tap the V icon to expand the menu
4. Now you can see the A icon, that is the language and input menu
5. Here is the menu for change the language
6. Choose and tap the new language you want to set.