Is it possible to make mountains yield?
In TerrainInfos.xml I added this:

<Yields>
<iYield>2</iYield>
<iYield>0</iYield>
<iYield>0</iYield>
</Yields>
(just whatever while testing - it would yield production of course)

But I cannot use the mountain at all in the city screen, whatever yield may exist...

Mountains are inherently unworkable. You need to find the tile info and change that as well as the yeild.

You'd need to dig into the SDK. Impassable terrain is set to be unworkable there.

At most I would give it 1 Gold, to represent money earned from skiers or mountain climbing parties. No production and no food would be possible.

Thanks for the help guys.

@kiwitt:
I can't see that it is any of your business what I would want to make it yield.