So, first of all we should make a difference between the processes of translation and interpretation. We can view translation as the process of creation of text, by turning an original or “source” text into a text in another language. Interpretation on the other hand is the process of creation of speech, by turning an original piece of verbal communication into a piece of verbal communication in another language. These two processes differ in their input/output, duration, subject, etc. The translation is a written process and usually involves high specialized subjects. As we know, specialized texts have their own terminology and do not contain idiomatic expressions. Also, translations can be reviewed and referred many times before the final translation is delivered.
The native proficiency in the target language is outweighed by the acquired proficiency in the source language and the technical knowledge. In the intellectual processes of review and references both have to compete equally and the management of available time gap between input and output is more dependent on their intellectual and other resources than on their native language. However, in interpretation, native speakers outdo the non-native speakers on all counts. Since they speak naturally in the target language, they are fast in translation and render the meanings accurately in the native idiom, without having to look up references for words and usage. Their pronunciation, stress and accent, which are an extra burden for the non-native speaker, would be perfect and well accepted and understood. Their task is made further easy when it is about non-specialized speeches. The only common area of competition is proficiency in the source language, where again native speakers are at an advantage if the source and target languages are nearly related like Kurdish and Persian or Spanish and Portuguese.
So the mother tongue is important, though not crucial for translation:)