Professional translators are writers, producing texts that read well in the target language. They are usually fluent in their source language(s) as well. But they are above all effective bridges between the languages they work in; they can render the message of the original text, with appropriate style and terminology, in their native language. Bilingualism is something else. Bilinguals speak two languages fluently, but are not necessarily good at moving information between the two, especially in writing. And many people described as bilinguals overestimate their communication skills altogether.
Lina’s, a pricey French sandwich chain, advertised for franchisees abroad with a text concocted by a self-proclaimed bilingual employee. Slogan: “Tomorrow, we will expect on your dynamism.” Response: zero.