Where a native minority imposes its language on an immigrant majorityWatch
1. Where a conquering force imposes its language on the native population. An example are conquistadors in South America which imposed Spanish and Portuguese on the native population, and they later became national languages of the countries. Another example is English which is derived from a Germanic language spoken by immigrants from northern Europe who migrated to England in the 5th century, and their language largely replaced the Celtic languages and Latin which were then used by the native population.
2. Where a smaller immigrant group adopts the language used by the much larger native population. An example are immigrants to Britain after 1945 who learn English. Second generation immigrants often have better knowledge of the language used by the native population than the first language of their parents.
3. Where a localised group of people revert to their traditional or ethnic languages from national languages. An example of this are satellite states that broke away from the Soviet Union where the citizens have changed from Russian to languages like Uzbek, Latvian, Armenian etc. since 1990.
4. Where an immigrant group adopts the language of a much smaller native population.
5. Where an immigrant group adopts the language of a much smaller native population that is not the national language.
Are there any countries where (4) has taken place? To a limited extent it applies to the US in the late 19th and early 20th centuries where large numbers of immigrants from Europe adopted English, although in reality English isn't the language of the native population of the US, and the US was very multilingual at the time with German in a strong second place along with Italian, Polish, Hungarian, and other European language speaking communities. Another possibility is that Some Gulf States have more expat workers than citizens living in them and many of them learn Arabic.
The most interesting case is (5) and I think the only country that this scenario has taken place in is Mauritius where immigrants from the Indian subcontinent who arrived in the late 19th and early 20th centuries (and now comprise about 80% of the population) adopted Mauritian Creole which was the language used by the then established population descended from African slaves from the 18th century (and now comprise about 15% of the population) when the official language of the country is English. The Indian immigrants never successfully imposed their languages on the native descendants of African slaves from the 18th century, or conversely, they refused to embrace them.
Are there any other cases of (5) in the world?