Обычно "If you know Russian..." не говорят.
Говорят "If you speak Russian", или "If you know how to speak Russian"
If you know how to speak Russian, you will be able to communicate with people from many parts of the world. For instance, Russian language is spoken in all countries of the ex-USSR block. Also, there are a lot of Russians living abroad. You will also be able to read Russian books and newspapers.
Knowing how to speak Russian will give you an opportunity to work in Russia. If you decide to immigrate to Australia, proffeciency in the Russian language will provide you with an extra credit.
The language will be useful when you meet people that speak Russian. Also, it will be very useful to read famous Russian authors using the original language the books were written in.
Without knowing how to speak Russian, it will be difficult to communicate with elderly Russian folks, since they usually do not know how speak English that well.