Non-English speakers reveal more knowledge of their own native language. For example, if these speakers make grammatical mistakes in english, they might reveal the grammatical rules of other languages.
By analyzing the patterns of mistakes that native speakers of two languages make in English, computers can discern whether two languages might actually be related to one another, as the structures of these languages are similar. Computer error analysis can also be useful from a historical linguistics standpoint. The mistakes speakers of endangered or dying languages make when they're speaking and writing in English can serve as an archaeological window into other languages and the history of language.
Non-English speakers reveal more knowledge of their own native language. For example, if these speakers make grammatical mistakes in english, they might reveal the grammatical rules of other languages.