I’ve recently come across news articles about English as the language of instruction in public schools in countries where it may not be the first language spoken by students, such as the Philippines and India – I believe this is the case in a few other Asian countries but don’t have current news articles to link to. I also don’t think it’s uncommon for French and/or English to be the language of instruction in some African countries (see this TEFL Logue post on Frananglais).
The presumed benefits are somewhat self-explanatory – students will become proficient in English not only in conversation but in academic subjects, they may fare better in the work force, etc. There is also some idea that having English as the language of instruction removes the connection between having money and having the chance to learn English.
However, the recent news articles I found appeared because many people find problems with instruction in English. In India, sometimes the worry is that non-Hindi-speaking students studying two second languages, Hindi and English, may have negative implications for their own language, and at the same time they are taking in too much to become proficient at either English or Hindi. I suspect that one connected concern is not that they will stop speaking their own language, but perhaps that over time, there will be fewer and fewer literary works in those native languages as educated people are trained to write in other languages.
One interesting point brought up in the article about the Philippines is that English as the language of instruction tends to benefit most those who already have a strong background in English – they can excel and students weaker in English feel intimidated or can’t keep up. The authors even assert that the English policy may be a factor in students with weaker English language skills dropping out of school, and in this way the policy may hurt the country in general more than it helps.
I don’t have much information to base an opinion on, but I think it is notable that there can be very real downsides and even negative implications of a policy that at first glance may seem beneficial. It remains to be seen what the outcome of all this will be.