I saw this interesting blog post on using “stepmother tongues”, i.e. on people who write in or translate to a language other than their native one.
I have worked on translations to Swedish with native Swedish speakers, but I doubt I would ever want to be completely responsible for any jobs to Swedish, because it is not my native language and I know there are things I would miss or be unable to translate as well as I could when working to English. I turn down such assignments when asked to take them on, explaining why. Likewise, I regularly write articles and essays in Swedish, but I would probably not want to write fiction in any language other than English. You just have a different feel for your mother tongue than you do for your stepmother tongues.
Some rare people are true bilinguals and can write in or translate to more than one language equally well. And some people do eventually feel comfortable and confident working in a language other than their native one. The post mentions Samuel Beckett and Milan Kundera as examples; Vladimir Nabokov is another one, as is Elias Canetti, though he did learn German from a fairly young age.
But I confess that I am suspicious when people profess “true fluency” in a multitude of languages and take on assignments requiring them to translate both to and from their native language. Many people do have stepmother tongues, but sometimes those stepmothers can be wicked and can make us think we are better at them than we actually are.