I don't know anything about pay levels. But I guess what your wife's cousin does is drive JCBs? Those machines with wheels or crawlers (is that the right word in English?) he drives? That would be a Bagger as lifefire pointed out and the job discription is Baggerfahrer.

I think he would need to know a smidgen of German (or Portugues or Italian or some eastern European language to be honest). I have worked with plenty of people in that position that only know little German.

Whether there is a demand for Baggerfahrer I don't know, sorry.