"I am buying half of the apples."
Translation:Kupuję połowę jabłek.
"pół" is more like the numeral 0,5. It doesn't really make sense with a plural noun. You can have "pół jabłka", "pół kilograma jabłek", but not "pół jabłek".
Granted, 'half of the apples' isn't the most intuitive thing to say... but as I wrote in another comment, it's like "half of the apples they had".