A qualified translator needs to be responsible and professional. They need to translate texts faithfully and accurately. Also, since the documents might be sensitive, translators need to be professional in maintaining confidentiality.
Translators most certainly require a solid foundation in the mother tongue and foreign language. However, this should be moderated by cultural sensitivity of the language and context. In effect, the translator is a cultural coordinator and arbiter between two different cultures.
Finally, translators are required to learn something about everything. For example, translators engaged in business negotiations need to be familiar with negotiation terms as well as international corporate business etiquette.
Therefore, you should ensure that your translator comes from a reliable source and has a firm track record.