Boyfriend who speaks a different language?

My boyfriend is here for 3 years on a visa. He speaks Russian and I'm not sure but I feel like he definitely has another girl or more that he talks to back home. He has the app Viber where he talks to his friends and family. I can't understand Russian but he is trying to teach me. Still, it makes me uneasy I can't understand what he is saying... Anyone else have this problem? Thanks in advance