How People Speaking Different Languages Understand Each Other

I recently watched a TV drama series from Zambia; Love Games recommended by a friend from Germany. And though the language of communication here is English, there are parts when the actors and actresses resort to their mother tongues like Bemba. In such cases, there are subtitles ready to redeem us the lost souls, in the sense of not understanding the language. There are words and pronunciations just like in the Ganda language used in Uganda and other languages under Bantu languages include; Kinyarwanda, Shona, Kirundi, Kiswahili as reported by www.britannica.com.