This question came to my mind because I saw some advertisment looking for English teacher in this forum. One of the requirement is that they must be a native speaker. So I start wondering how these people define native English speaker. 

A very controversial example is Singapore. Singapore was a British colony. After it gained independence from Britain, it still uses English as one of the official languages. Even though I say "one of", English is basically the only functioning official language. 

People in Singapore today speak only English at home, they are mostly of Chinese and Indian ethnic. The media language in the school is also English, all their textbooks, except those language books, are in English. Singaporean will need to sit GCE O-level in secondary school and GCE A-level in junior college, which are also in English. 

I once read in a newspaper that western people do not consider Singaporean as a native English speaker. However, many Singaporean can only speak one fluent language which is English, if they are not considered a native English speaker, then what is their native language? Chinese or Tamil? If they can't even command Chinese or Tamil, how can they be considered a native speaker of these languages. 

 I couldn't agree more, many people today speak English as their first language. The problem is that many people today still has the perception that English belongs to white man living in UK, USA, Canada, Australia and New Zealand. Therefore only these people living in these countries are considered a native speaker. 

By the way, is there a different between these two terms "mother tongue" and "first language"? Although it is used interchangeably, I still ponder if there is a significant different. For example, a person mother tongue can be French, but in his whole life, he uses English as his major language to communicate, so, wouldn't that make his first language English?