Lithuania's official language is Lithuanian.
Lithuanian universities provide degrees in both Lithuanian and English. Postgraduate degrees are more commonly taught in English. If you want to study in a language other than your native tongue, you will very certainly be required to show proof of your language skills. If you don't meet the required standard, your institution might be able to assist you improve by offering language classes.
Even though you can study in English, you should strive to learn as much Lithuanian as possible. Practicing with locals and other students is an excellent approach to do so. This is a valuable skill to have on your CV or resume!