Why "două" not "doi" ?
It depends on the gender of the noun. Doi is used with masculine nouns and două is used with feminine and neuter nouns. Pâine is a feminine noun so două is used here.
Numbers higher than two do not make this distinction.
Felii means slices, not loaves
Loaves of bread is "felii de pâine". Transation is not correct
No, that is slices of bread.
Transation is " two breads"
"Bread" isn't a countable noun in English.