She is the only First Lady that did not have English as her first language.
While she is not the first naturalized First Lady (meaning she was not born in the United States), she is the second after Louisa Adams, wife of President John Quincy Adams, and the first one that does not have English as her first language. She was born in Slovenia, which makes Slovenian her maternal language, but that does not mean that she cannot speak English. She has proven time and time again how fluent she is, and not many pay attention to the fact that she is a polyglot.
Melania Trump speaks more languages than any other First Lady, as, in addition to Slovenian and English, she is reported to also speak French, Italian, German, and Serbian! No matter what way you look at it, it is an impressive roster of languages, as they are all hard to learn, let alone be able to converse in them!