Solution

We have the events S (being Swiss) and GS (German Speaking)

The probability for a randomly chosen person to be Swiss:

If somebody is Swiss, the probability of speaking German is 0.64:

So the probability of the earthling being a German speaking Swiss, can be calculated with:

inserting the values from above gives us:

and

So our aliens end up with a chance of 0.07168 % of getting a German speaking Swiss person.