So, you know how when you go to school or go outside to play, you might see different people who look different from you or speak different languages? Internationalism is when we like having all those different people around and think it's cool to learn about their cultures and ways of life. We try to treat everyone fairly and understand and respect each other's differences.
On the other hand, Russification is when one group of people (in this case Russians) try to make everyone else in their country act and believe exactly like them. They might try to make everyone speak only Russian or have the same customs and beliefs as them. This can make people feel like they're losing their own culture and identity.
So basically, internationalism celebrates diversity and differences, while Russification wants everyone to be the same.