It is considered vital that translators translate into their mother tongue, as we will never truly master a language we learn later on in life. But hey, wait a second; even being a native speaker of a language does not make you an expert.
If a translator isn’t a skilled writer or doesn’t have a thorough knowledge of their own mother tongue, it does not matter how well they may understand the source text as they will never be able to create a quality target text. It is essential to write perfectly.
Therefore, knowing your native language perfectly is as important, or perhaps even more important, than having a good command of a foreign language. It is a big mistake to think that simply being a native speaker of a language qualifies you to translate.
Translators are communicators who need a wide vocabulary as well as excellent knowledge of their mother tongue. They should also have a great command of the subject field they are translating. A professional translator should not accept a project if he does not feel comfortable with the topic or if it is about a field that they do not thoroughly understand.
Through translating several texts in a particular field, a translator can become an expert in that field. However, there may be occasions when inaccurate solutions or terms are used which may not be appropriate in that particular field. Therefore a proofreader specialised in that field should always do the final reading of the entire document to ensure that everything is accurate.
It is very important that the person who proofreads the translation also has experience in translation and is a native speaker of the target language, as again, simply being fluent in a language does not qualify you to translate or proofread.
Like any other profession, translation requires training, practice and experience.