Industrialization of the 1900s and 2000s, especially in the English speaking countries, caused individual countries to become richer.
Then they were able to purchase things from all over the world.
The non-industrialized countries improved their economies by selling their natural resources.
To be able to sell their products the non-Enlgish speaking countries had to learn some English.
Now, English is the dominant default language of business and trade.
So, it's all about the money (plus many of the other comments and reasons previously stated)
Have you ever thought before why English seems to be the most important language all over the world?
That's how I would ask people to share their opinions :)
The first most spoken language is Mandarin, then Hindi slightly before English & Spanish. Therefore, you would wonder why English it's so important and why even in your question you wrote "the first language"? In my opinion, it started with colonization spreading English all over the world, though Spaniards also had quite enormous colonies. Nevertheless, Great Britain always had a lot of money to invest and that's how they became powerful by investing in colonies. Spaniards didn't have means to rule the world so British used a situation and nowadays you can see that the most knowledgeable and skilled researchers go to work for the powerful countries such as the US, Germany, Japan and South Korea - all of them approved English to be a science-language. In the past, nobody was really interested in learning German, Japanese and Korean, that's why English become powerful & it is said to be a business- or science-language of the world. In the past Europe seemed to be in the centre of the world and specially France had quite good position in the whole world. Telling the truth, if it wasn't for British colonization, I would bet that nowadays we all would love to learn French :) Anyway, as I said the British had quite big influence in the English language spreading and as the science-friendly (or business-friendly) countries had a choice between English, German, Japanese and Korean - obviously they chose the most spoken language. I'm not sure how are such countries as Russia, the UAE, Saudi Arabia, Qatar with it's technology and science but I would say they started too late and they need to follow English-business line right now. Basically, I would say English won because of its simplicity - imagine coding everything in Kanji, Hiragana or Hangeul - it's easier to use Roman alphabet.
I think not in all country English is the first language because mother tongue is considered as the first language and yes,in most of the country English may be considered as 2nd language as in My country.
The reason why English is so popular is explained by @Ewa nicely with facts.My country was the hub of British colony nearly about 200 years.So it is obvious that spreading the language among the people is inevitable.And most of the freedom struggles were aware of English language and use it as a weapon to fight against them.There were French,Portuguese and Dutch colonies also here but they couldn't sustain as British colony did.But still, in some part of my country, people say French and Portuguese.But most of the people aware of English language if can't speak but understand a little:)
So this effect must have influenced the rest of the countries where British colonies were there.And for that reason, English is an international language,a lingua franca and a global platform for the people in workplaces,institutions and last but not the least in language exchange between people belong to different country and culture.
Please correct me If I lacked in somewhere.Thank you:)