Translation companies with highest standards of professionalism, the industry's 'big players', often on their websites refer to ISO certifications or their own quality management procedures which in most cases include the 'two-man rule'. The phrase «we only work with native speakers» is rarely found on these renowned agencies' websites, while it almost dominates those of countless semi-professional “language service providers” (for which the term 'CAT tool' is fully unknown). It seems the emphasis on the term 'mother tongue' is an attempt to portray a lesser service in a positive light.
What does a statement of working 'with native speakers only' actually mean and why is there a tendency to perceive such a statement as a sign of quality? Generally speaking, the so-called mother tongue principle means that a translator always translates texts into his or her mother tongue. This method, although wide-spread throughout the industry, is not self-evident at all. First, the question of who can be considered a mother tongue speaker can quickly arise. The term indicates that it is the language spoken by one's mother (hence the German joke with two mothers when somebody says he has two Muttersprachen). The English language has besides of 'mother tongue' also the term 'native language' or 'native speaker'. A lucky distinction that is not given in German.
Many people speak from their early years on two and more languages; others may move as teenagers or in their early twenties to another country, where they spend years and decades of their lives and speak the respective language without any foreign accent. This can result in their countries of origin actually becoming, as time passes, foreign. Have they not become native speakers of their new homeland's language?
There are also good reasons to ask in what way mere 'possession' of a mother tongue is decisive on whether a person can translate contents adequately into a different language. Is specialised knowledge, well-trained handling of tools such as terminologies, glossaries, and translation memory tools as well as profound translation skills not more relevant? And: does the mere attribute 'mother tongue' say anything about the education level, the vocabulary or the writing skills of a translator?
The importance of the mother tongue principle is systematically overvalued, while, from my experience, there are two conditions for a translation to be of a good quality. On the one hand, a good translation has to reproduce the source text in terms of its content, style, the author's intention and the recipient group impeccably. To achieve that, you have to understand the source text completely, which is a strong argument against rigid application of the mother tongue principle. On the other hand, a good translation should read as if it were not a translation but an original text. In other words, a good translation is one that gives the impression of not being a translation.
Good proficiency in two languages is a pre-requisite for both of these conditions to be met. Misunderstanding the source text will always result in an imprecise translation. If the knowledge of the target language is not strong enough, then mistakes in the translated text can be expected. Only very good (or even better, perfect) knowledge of two languages makes high-quality translation work possible while it is of a secondary importance which of the two languages was spoken in early childhood. In Germany, this requirement is met by passing respective State Exams for translators. Contrary to what the free market dictates, you are tested – and rightly so – in translating into both the 'native' and the 'foreign' language.