Mongolians speak Mongolian, a language that sounds unlike anything I had ever heard before. There are two ways to write Mongolian, Cyrillic and script, the former having been introduced during the 20th century Soviet occupation. Script is still used in formal capacities and at the government level and is still taught in schools. Also note that to the far west of Mongolia, there is a province called Bayan-Olgi that is largely Kazahk in population and culture. The volunteers who are sent there will have to simultaneously learn two languages – Mongolian and Kazahk.
There are a few letters in Mongolian Cyrillic that don’t exist in the Russian alphabet. I’ve always been able to tell right away if a word is Russian or Mongolian. It’s hard to explain but letter combination and arrangement is very different between the two languages.
Mongolian script is incredibly beautiful and it’s amazing to watch someone write in it. Many PCVs get script tattoos before they leave to commemorate their experience here and it’s something I’m definitely thinking about doing as my first tattoo.
In terms of difficulty, I’d say Mongolian is up there and can be especially difficult for speakers of Roman or Germanic languages. Sentence structure is very different and it takes some getting used to, especially when you’re trying to speak. However, based on the tiny selection of people I’ve talked to, those who have learned/know Japanese or Korean often have an easier time with it. Also knowing Russian in Mongolia is helpful (and at times has proved advantageous) but it won’t help much in actually learning Mongolian.