Every rational language can be represented by (at least one) rational expression, and every rational expression represents a rational language. That means for any rational language, you can find the equivalent expression (though it might not always be a simple task). In fact, there are algorithms that convert any NFA into an equivalent rational expression and vice versa, proving their equal expressive power: for example the McNaughton-Yamada-Thompson algorithm for converting a rational expression to an NFA and Kleene's algorithm for converting an NFA to a rational expression.
Rational expressions and languages are usually referred to as "regular" in English, but the word "rational" is also used. Be careful with the term "recognizable", however, because it is usually taken to mean "Turing-recognizable" in English discourse in automata theory – these languages form a much larger group than rational languages, and include many non-rational languages.