The "Restore Purchases" option is available on both Android and iOS devices, but its function is different in each case.
On Android devices you can do so only to restore your purchase of the old app in the new one and only in case of apps that feature a single language course.
On iOS you can restore your purchase of the new app that features all 34 language courses if you installed it on another device.
Note: You can restore your purchases between Android devices or iOS devices. You cannot restore them from Android to iOS and vice-versa.
How do I restore my purchases?