Comment ca va? google Translate says is: "How Are You?" in English: However,

a more realistic translation might be: "How's it going?" So, I think How are you is inaccurate but does give you the basic sentiment.

Where as a more realistic statement in English regarding Comment allez vous?

would be how are you Going? if you directly translate it. But, in the end "How are you doing" is an equivalent in English even though this is not what is really being said in French directly.

It's because people say things differently in different languages because the languages started differently.

So, even though French and Ialian, and Spanish for example are all romance languages, German and English are not romance languages. So, French Italian and Spanish will be more similar and German and English and Scandinavian Languages will be more similar. 

For example, English is a Trader's language for buying and selling goods between countries off of ships because England is an Island nation so you needed a ship or a boat to even get there from the mainland of Europe.

So, English for example contains parts of French, German, and Scandinavian languages the most as well likely as Gaelic and other languages as well.

So, English is likely one of the best languages on earth to conduct business in which is why it is so popular around the world for conducting business between nations.