Complex sentence: The Belgian authorities intended to have him serve at least half of his sentence in Poland, thus ensuring that he would have to serve at the very least 10 years because Polish criminal law demands that a convicted criminal must serve at least half of his sentence.
Split into simpler sentences: The Belgian authorities mean to have him serve at least half of his sentence in Poland. This would ensure that he would have to serve at the very least 10 years because Polish criminal law demands that a convicted criminal must serve at least half of his sentence.