In a worst case you can end up with the Scots Wikipedia situation, where some power editor created a bunch of pages using an entirely fabricated, overly stereotypical language and that influenced what people thought Scots actually was.
This is one of the examples we keep in mind and that's also why we can't 100% trust public dataset labels. This motivated us to train a Language IDentification system for all the languages we wanted to handle in order to build the monolingual dataset. More details in the paper ;) Or here, if you have questions