* Defining "second language" is tricky: Does it include any language other than English, even if it's only a few words? What about fluency levels?
* Data collection methods vary: Different surveys use different definitions of "second language" and have different sampling methods.
* The US is a diverse country: Language proficiency varies significantly across regions, ethnic groups, and age demographics.
However, we can look at some estimates:
* The US Census Bureau: Reports that about 21% of US citizens aged 5 and over speak a language other than English at home. This doesn't necessarily mean they are fluent.
* The Modern Language Association (MLA): Estimates that about 20% of US citizens are proficient in a second language. This is based on data about language instruction and use.
It's important to remember that these are estimates, not precise figures. The actual percentage is likely somewhere between these two figures, but it's impossible to say for sure.