A good translator of English (but not only) should primarily be characterized by a high level language skills. It is not only about the excellent use of a foreign language (though it undoubtedly is also very important), but also showing competence in their mother tongue. Why translator of English should possess this knowledge? If he is not be able to speak and write in accordance with the rules of style, spelling and grammar of the native language, even if he understands the text, which is to be translated in every detail, he will not be able to professionally translate it. None of us would like to receive a translation that is simply stuffed with stylistic, spelling or grammar mistakes.
Not everyone who knows English can be a translator. This is because if a person is not able to write and speak freely, their vocabulary is probably not rich enough and, his translations are not worthy to be called a professional. You should also pay attention to the person's psychological predispositions. Remember that the translator must often do their work under the time pressure. Therefore, he should perform well in a stressful situation and be available. A good English translator can create a professional translation regardless of the time pressure and the length of the translated text.