The vast majority of professional translators agree that translators should only translate into their mother tongue, but most translation clients are completely unaware of this fact. While there is a rare breed of bilinguals who have a gift for languages and excel at reading, writing and speaking in two languages, the majority of translators communicate best in their native language.
As any language teacher will tell you, language proficiency is a spectrum and fluency can be defined in multiple ways. For example, there is the person who can communicate in their foreign language while travelling, immigrants who have been obligated to learn a new language to get by in their new country, professionals who use a different language in their work and, as a last example, people who were raised speaking two languages. Nevertheless, just because you were raised speaking two languages “fluently” does not mean that you can write equally well in what is known as the “target language,” which is an absolutely essential skill that translators must have.
As I’m sure you would agree, the ability to write well requires a mastery of grammar, a knowledge of idiomatic expressions and, of course, a large vocabulary. Research has moreover shown that the total vocabulary size of monolingual and the bilingual children is not statistically different, but that the vocabulary of bilingual children is split between the different languages.
Since most people write best in their native language, it is a common practice in the field of translation to have your work revised by a native speaker when you translate into your foreign language. Even better if translators working into their native language, for their part, have their work revised by another native speaker to polish their translations and ensure that they are impeccable.
Given all of this, it often makes more sense to work directly with a native speaker to avoid the extraneous cost of reworking a rather imperfect initial translation, or worse, not reworking it at all and paying the price for a poor translation. If you’re not convinced yet, keep your eye out for bad translations. They are almost always the result of a translator working into their weaker language, and they will be all the proof you need.