Regardless of disability, the ADA requires that a variety of public and private services must provide an interpreter so that communication is accessible to the deaf, deaf-blind, and hard of hearing. Often companies are uncertain about when to hire a professional interpreter; especially when they know someone who is conversationally bilingual and willing to help out. Unlike bilinguals, interpreters are trained to interpret from one language to another accurately, effectively, and with proficiency. If you use a bilingual in your office or workplace, there is no guarantee of quality, accuracy, or confidentiality of information.
Interpreters know more than just two languages; they are also knowledgeable about the cultures associated with those languages. In general, using non-professionals is against the law. Without a professional interpreter, miscommunication is a risk. If you work for a company, access to language and information should be equivalent for all of your employees. Non-professionals can possibly misunderstand information, and in turn transmit wrong or misleading information. Deaf, deaf-blind, and hard of hearing individuals are not necessarily fluent in English, therefor writing back forth may not be ideal for specific settings.