Some airlines seem to like people to come up from within, so a summer job helping out in the airport, either a customer facing position like a temporary/part time GA or below the wing handling bags, might be viewed very favorably by the airline. Also look for any aviation/travel related internship opportunities.

In addition to business (finance, accounting, marketing, human and labor relations, etc.) majors, relevant academic areas would include specializations of logistics and supply chain analysis, maintenance planning, networks, scheduling, etc. within operations research and industrial engineering, where you also find a few specialists in yield/revenue management in some universities. I strongly agree with the advice to get a good background in probability, statistics, and stochastic processes as well as computer programming and optimization techniques.

I would assume that LH requires near-perfect fluency in German and English. Most foreign carriers would require English and expect other foreign languages.