The terms translator and interpreter are commonly confused, but the distinction between the two is quite simple. Translators work with written material and interpreters render spoken communication. Both are language specialists and should be knowledgeable in the subject matter at hand.
Interpreters: Accurately render speech from one language into speech of another language. Translators: Accurately render written materials from one language to written materials of another language.
Translators can produce idiomatically correct, accurate work more quickly and easily when translating into their native language. Some translators have lived in another country so long that they have achieved a high level of competence in the new language, and prefer to work into it. This language becomes their dominant language. Note that a person cannot have more than one dominant language, any more than s/he can have more than one native language. Some translators claim to be bilingual, to have grown up with two equally developed languages. It is well to be skeptical of such statements until they are proven.