如何翻译"谁知盘中餐,粒粒皆辛苦."(英语)

Note that "知" does not necessarily mean "know".It also means "realize".
The phrase "谁知" is to call for attention.It may be omitted when translating as a motto.Thus it reads either
Each grain in your dish is packed with sweat.
Or,
Each grain in your dish is a fruit of hard labor.
However to translate the entire poem,it is better to include "谁知" for coherence.Thus it will read:
How many of you realize
that each grain in your dish
is a fruit of arduous labor?

Look at the food on our plate, every grain of which is from hard work