Most often, this is their native language. Most people, including translators, truly master only one language in a lifetime.
True bilingualism is rare indeed.
Translators are highly skilled linguists who generally enjoy learning foreign languages, and they do so with relative ease. They learn to read foreign languages, and in many cases even learn to speak them, but it is very difficult la learn how to write a foreign language so that it comes out error-free and looking like a native wrote it. Therefore, it should not be assumed, for example, that a translator who churns out superb work from French to English will also be proficient from English to French. Of course, there is an exception to every rule. In some language combinations, for example, languages of limited diffusion, a native speaker or the equivalent may simply not be available.