As of ME2 It has only been 30-some years since humanity first explored outside it's own solar system, so Zaeed had to have been born on Earth rather than on a colony world. Since he has a British accent, it's safe to assume that he was born in the UK and his nationality is British. Since he has an Arabic first and last name, it's safe to assume that his ethnic origin is either fully Middle-Eastern, or Middle-Eastern on his father's side of the family.
For those that are confused as to why a person with a British accent has a Middle-Eastern name, there is immigration to the UK just as there is immigration to US, Canada, Australia, or other Western nations.
As an example, here is the demographics for modern day London:
According to the Office for National Statistics, based on 2006 estimates, 69.4 per cent of the 7.5 million inhabitants of London were White, with 58 per cent White British, 2.5 per cent White Irish and 8.9 per cent classified as Other White. Some 13.1 per cent are of South Asian descent, with Indians making up 6.5 per cent of London's population, followed by Bangladeshis and Pakistanis at 2.3 per cent each. 2 per cent are categorised as "Other Asian". 10.7 per cent of London's population are Black, with around 5.5 per cent being Black African, 4.3 per cent as Black Caribbean and 0.7 per cent as "Other Black". 3.5 per cent of Londoners are of mixed race; 1.5 per cent are Chinese; and 1.9 per cent belong to another ethnic group.[5] In January 2005, a survey of London's ethnic and religious diversity claimed that there were more than 300 languages spoken and 50 non-indigenous communities with a population of more than 10,000 in London.[6]
http://en.wikipedia....phics_of_London