There are lots of different dialects and languages used in Afghanistan - the Mujahideen were not all one ethnic group.

The most widely spoken language in Afghanistan is Pushtu,so more Mujahideen perhaps spoke that than any other language.

Probably 'Dari' the Afghan version of Persian it's the most widely spoken language in the country even though Pashtuns are the country's biggest ethnic group speaking Pashto an Iranian language. Afghanistan has several similar ethnic groups, mostly Pashtun and Tajik but several smaller ones. So they can talk to each other most speak Dari beside their ethnic language.

The Soviet invasion was not like the American invasion of 2003. They did not try to change the country or knock off Muslims on purpose. They just wanted to prop up the Communist party at that time.