Dear Ms. Hout Sunny- Only use the family name in an English letter in the greeting, but if this goes against Cambodian custom, then leave both names.
my interest for the Sale Executive position- interest in
from National University of Management (NUM)- from the...
sale man- salesman
career goal meet your requirement- career goals meet your requirements
mean time- meantime
Thank you for your time and consideration.- Thanking you in advance for your time and consideration, I look forward to hearing from you.