There are six or seven languages spoken in Nepal, Dzongkha being only one of them. Actually, English is the official teaching medium. If one uses the term "Bod Ked," Tibetan language, most Bhutanese will understand that as colloquial Tibetan. If you mean the Tibetan used in Dharma texts, prayers, and pujas, then they refer to that as "Chos ked," Dharma language. Young people all speak English; and many, many people (if not most) also speak Nepali. While Dzongkha is obviously part of the Tibetan language family tree, there are definite differences in grammar, vocabulary, and pronunciation. If you try speaking Tibetan in Bhutan, some people will understand you and some will not.
Pema Chophel པདྨ་ཆོས་འཕེལ