What is the level of proficiency required?
Do you mean what do schools aim at or what level is required to study translation?
Those who did seven years of English at school (the norm in my generation) will have a B1/B2 level knowledge. That's just enough to be able to watch Netflix episodes in the English original and hold a conversation on a variety of topics. Those who have spent some time abroad or used English at university (lots of textbooks are in English, my sister even wrote her PhD thesis in English because all the terminology in molecular biotechology and most of the literature is in English anyway) will have a C1 level (fluency).
I meant what schools aim at. So it sounds like it's less than fluency. Does this include the second language, for those who didn't go on to become a translator?
Yeah, you usually don't get full fluency with what schools can offer. Those who do study a second language usually do so a few years after the first one, so it'll be even less. But that is still much better than no second foreign language at all. It's a foundation, usually enough to hold a simple conversation and get by as a tourist.
I forgot to link to the definitions of the Common European Framework of Reference to explain what is meant by "B1" etc: Common European Framework of Reference for Languages - Wikipedia
To be fair - in this context, "fluency" or "proficiency" means you can read and understand both Shakespeare and legal texts. So, probably something not all native speakers will be able to do.