"Natural Gas" is 97% methane, so it's probably most direct to just refer to it by it's primary component...

In Poland, it's called "Earth gas" or "ground gas".

Also in Dutch: aardgas

And German: Erdgas.

In some countries it is also called "city gas"

City gas was obtained from distilling hard coal or, later, from gasoline. So not the same thing as natural gas.

City gas is actually something else, it's coal gas.

Yes, but it's still called that way

It has become popular to refer to it as "fossil gas".

oil gas? carbon gas? fuel gas? fossil gas?