Complex sentence: A major breakthrough in the company history was the take-over of a majority of the Berlitz Language Schools after its going public in 1989, in 2001, Benesse Corporation acquired 100% of it and made it a private company once again.
Split into simpler sentences: A major breakthrough in the company's history was the acquisition of a majority stake in Berlitz Language Schools, which had gone public in 1989. In 2001, Benesse completed the take-over by acquiring 100% ownership of Berlitz and making it a private company once again.