bresych is actually a plural noun. It is best translated as 'cabbages' or as 'cabbage/some cabbage/some cabbages' depending on the context:
- Dw i eisiau prynu bresych - I want to buy some cabbage (as in some indefinite amount to use at home)
- Ga i fresych, hefyd? - May I have some cabbage as well?
The singular is bresychen (a cabbage):
- Dw i eisiau prynu bresychen i fy mam - I want to buy a cabbage for my mum (a single cabbage as that is what she has asked for, perhaps)
This sort of thing is very common when discussing food, plants, trees, etc - anything which tends to come or be discussed in bulk as well as as individual items.