The consensus of opinion among translators is that you should always translate back into your mother tongue.
Mother tongue means your first language, the language that your parents spoke to you and that you were brought up with and invariably that includes the language that was surrounding you where you were brought up.
The idea is that if say for example you were born in Britain but studied Spanish no matter how good you have become at speaking, reading and writing the Spanish language you should always translate from Spanish which is your studied language back into English which is your native language.
Generally speaking this is a good idea but being dogmatic can be problematic.
Let’s look for the sake of argument at a hypothetical case.
A hospital in a Spanish speaking country wants to have an NHS published medical document in English translated into Spanish.
Following the mother tongue advice, it would be logical to seek a Spanish native speaker from the relevant Spanish speaking country to carry out the translation.
What might happen?
We find a university professor who is a native Spanish speaker and who loves Britain. He knows a lot about the history of Britain and loves British theatre television and films. He has even taken part in an amateur production of a Shakespeare play; so he really understands the English language.
He knows nothing about medicine or hospitals, however.
Should we give him the translation job?
Here is another option:
A girl is born in Britain of a Spanish origin family. She is very studious at school, gains lots of high grade A Levels and goes on to university where she studies medicine and passes with first class honours with distinction.
Her language at home was Spanish when she was growing up because her parents felt it was very important to keep their Spanish culture alive.
Her language at school (perhaps grammar, perhaps private) in a primarily British area and at university is however English as is the language of her medical texts.
Now an adult and working in the NHS as a doctor she uses her Spanish language when seeing Spanish patients if that is what they prefer. She is involved in helping Spanish medics in Britain and has worked with Doctors Without Borders in a Spanish speaking country.
Her medical Spanish is an active language. She is in fact practically bi lingual especially with reference to her own area of expertise: medicine.
She has what is sometimes referred to as subject specific knowledge.
My thoughts are that despite the fact that she was born in Britain, brought up in a very British environment and uses the English language professionally she is the ideal candidate for translating a medical text from English to Spanish.
She will almost certainly know and understand just about everything in the specific area of medicine to be translated and she is surrounded by her colleagues who may be happy to provide medical opinions and she has access to highly specialised dictionaries and other resources.
So, what is more important: where you were born or who you have become?