You are offered $100 million but your mother tongue instantly changes to another language (<10k speakers). You lose any additional languages you have learned since birth.

The language chosen is random. You are allowed to hire a translator (if you can find one). You can be unlucky and get stuck with a language with fewer than 10 speaks , like Ainu.

You will instantly become fluent in the language. It will appear to you that you are thinking and speaking in your original language but to everyone else it will be appear as the new language

You also lose your ability to read and write in your mother tongue.

You can learn a new language but you start from the beginning.