Debates about immigration often lead to the topic of language. Donald Trump believes that only English should be spoken in the United States. In August he backed a bill that would evaluate prospective immigrants on a points system, with more points given for demonstrating English language ability. “Whether people like it or not, that’s how we assimilate,” Trump said in 2015.

The reality of language assimilation is more subtle. When people move to a country where a language other than their own is spoken, they have many incentives—cultural, economic, and social—to learn to talk like a local. That is probably why English is one of the most popular languages in the US on Duolingo, the world’s most popular language-learning app.

For these English learners in the US, Spanish is by far the most common native language. There are 9.1 million users studying English in America, and 7.8 million of those are learning it via Spanish, according to data provided to Quartz by Duolingo. In fact, in Miami more people are studying English than any other language on Duolingo. It’s the only major city where that’s the case, but the proportion is high elsewhere, too.

The 7.8 million users in the US learning English via Spanish make up 13.5% of Duolingo’s overall American user base, a company representative said in an email. That’s a big number, and proportionally it’s pretty close to the percentage of Americans that are Spanish speakers, probably about 40 million by now, or around 12% of the population. We don’t know how many of these users are recent immigrants, or whether they have been in the country for decades, but they are all trying to improve their inglés.

But there’s another point to be made on the topic of assimilation: many Americans are assimilating to the newcomers by learning Spanish. It is far and away the most popular language on Duolingo, the top choice for users in New York, Chicago, Los Angeles, and many others. In these cities, Spanish is more than twice as popular as the next competitor, French. Even English speakers, it seems, don’t agree with Trump that English should be the only language spoken in America.

The best way to learn a language is to move to a country where it is spoken. For an English learner, Duolingo can help, but it is no replacement for moving to America. Not letting in people who don’t already speak English is, therefore, a bit of a catch-22. Its overall effect will likely be to reduce the total number of people who are fluent in American English.

Finally, immigrants usually want to assimilate. The alternative is to feel like an outsider and miss out on job opportunities. Latinos in the US are increasingly using English and not Spanish, even at home. And, in Sweden, the most popular language on Duolingo is…Swedish.