Duolingo uses a Latin transliteration by default. Currently, Duolingo does not have built-in keyboards for languages that do not use the Latin alphabet. These languages include, but are not limited to: Greek, Hebrew, Russian, Ukrainian, etc.
To install language-specific keyboards on your device(s), please follow the instructions below.
Open the Control Panel, then go to Clock, Language, and Region. There, select Change keyboard or other input methods.
Click the Change keyboard button and add any languages your heart desires. You can have more than one keyboard layout for a language! To switch between languages, a keyboard shortcut is normally used (Shift+Alt by default, but you can change that). A phonetic keyboard can be found here. Windows 8 and higher have their own "mnemonic" layout (not without flaws, though)
Check the Microsoft site for a more thorough guide.
Open the Apple menu and click System Preferences. Select Keyboard and click the Input Sources button. You can add languages there. Mac has a phonetic one right there if you need it.
Also, check the option Show Input menu in menu bar. That is it. The default shortcut to switch languages is ⌘ + Space
Go to your Settings and find the General tab. Select Keyboard→Keyboards and click Add New Keyboard...
Go to Settings and find the Controls tab. Select language and input. Click the settings icon next to the keyboard, then click select input languages.